HP 435B Teardown

[amc184]

I bought this HP 435B from eBay with the intention of reusing parts of it in a project.  It's a really nice, well built unit, but without the power sensor they're fairly worthless.  I got this one for about USD25, including shipping.  Physically it's pretty tidy, and it probably works (not that I care).





It's constructed of two cast aluminium side frames.  Folded aluminium braces sit between these, and painted aluminium panels screw to the outside. 









The main riser board, A4, contains most of the actual meter circuit, as well as the power supply.  There are five amplifier stages, as well as support circuitry for the sensor.  The grey box appears to be potted, and contains the auto zero circuit.  The power supply can work from mains or a 28.8V Ni-Cd battery, and includes charging circuitry.  Mine doesn't have the battery option fitted.



The small board in the can behind the panel meter, A3, provides the reference output for calibrating the sensor.



The only horizontal board, A5, connects the other boards together.  It also holds the two range switch attenuators, made up of two ADG201 CMOS switches and some precision resistors.  Each group of precision resistors is surrounded by a grounded guard trace.

[dave_k]

Very nice! What project did you have in mind .. ?

[sportq]

SInce it was made in the UK ('U' in the serial number) I was desparately hoping to see a date code in the 1987-1988 range as there's a good chance I might have done the factory verification and calibration on it. The chips I can see make it mid-1997, I can't believe they were still making them then. I thought it was being phased out when I worked on them. We also made the 436 and 437 meters on the same line too.

Pete

[wertyq]

very nice

[dom0]

Maybe it was a lower-cost alternative to the 436/437/438 meters.

By the way, does anyone know the exact date the 3400 was discontinued?

[amc184]

Rather unoriginal project, I'm making it into an alarm clock with a nixie display.  I wanted to make it out of an old piece of HP test gear in this upright form factor.  The 435B was cheap, and the controls suited what I wanted.  The power button will stay as is, the range switch will be the main UI control, I'll probably replace the 'calibrator %' switch with a potentiometer for volume, and the 'zero' switch will be the alarm cancel switch, as well as part of the UI.

This unit is the 2732U revision, made in at least as late as 1997.  I'm not exactly sure when these first came out, the service manual I have says at least as early as 1980.  Interestingly, it also says these were made in both the US and UK.  I'm not sure if it was kept around for its lower cost, it could have just been because it suited some applications.  An analogue meter can be really useful at times, especially for unstable signals.

I wanted to do a quick teardown before this thing comes apart.  This must have been a common unit, but I didn't see much info on it on the net.

[G0HZU]

Shame you are not in the UK because I would have made an offer on the analogue display meter part of that power meter if it was surplus to requirements...

I have the older HP431C meter and I 'think' your meter dial would fit it. Over time the microscopically thin 'transfer' layer that holds the text peels away from the white background and can jam the needle movement. Mine has done this several times now and this is a common problem on similar meters made in the 60s and 70s.

However, I usually use the 'recorder' output at the back into a DMM for accurate measurements so I can still use the meter.

[G0HZU]

This must have been a common unit, but I didn't see much info on it on the net.

An analogue meter can be really useful at times, especially for unstable signals.

The HP432 and HP435 meters were very popular when working up at microwave frequencies where something was being adjusted or 'peaked' in power. Obviously, the analogue display was very powerful here.

IMO the one to have was the older HP432A with the HP478A thermistor head. The HP435 supports the later (modern) thermocouple type head but the HP432A with HP478A head is the real star because it uses a closed loop system for calibration and can be very accurate and maintain this accuracy for very long periods. The HP435 needs its internal 0dBm reference because it uses an open loop cal system.

Most people either want the older HP432A or the HP power meters with a digital display and GPIB so the HP435 is kind of lost in the middle and isn't in much demand these days.

[amc184]

The analogue meter is yours for the cost of postage if you want it, I won't be reusing that part of the unit.  PM me if you're interested.

The HP432 and HP435 meters were very popular when working up at microwave frequencies where something was being adjusted or 'peaked' in power.

That's exactly the type of thing I was thinking of.  Thanks for the info.

[johnh]

So what happened to all those power sensors?

Blow up/lost or are they reusable with different model meters?

[Vgkid]

^^^ I've always been wondering the same thing, I'm guessing blown up.

[MarkL]

The 435B uses the 8480 series power sensors which continue to be supported to this day on the newest meters from Keysight.  They must be at least 30 years old by now and a set of them for different sensitivities and frequencies can represent quite an investment.

And they *are* easy to blow up.  Don't buy any used ones unless they are returnable.

The reference oscillators I've seen in these vintage meters have excellent stability.  I would at least salvage that.

EDIT: The 8480 sensor series started appearing in the 1973 catalog with the 8481A.  So it's at least 42 years old.  Now that's long term support.

[Electro Fan]

Nice looking meter and nice photos.

Slightly off topic....

Where on the continuum from older not so expensive power meters (such as the one in the OP) toward something newer and perhaps more expensive might be a sweet spot of value for an RF power meter (and sensor?) if someone wanted to measure/confirm power when using a 3GHz signal generator with a 3GHz spectrum analyzer?  Any favorite HP or other models?  Any preferences on digital readouts vs. analog meters?  Thx

[Electro Fan]

This must have been a common unit, but I didn't see much info on it on the net.

An analogue meter can be really useful at times, especially for unstable signals.

The HP432 and HP435 meters were very popular when working up at microwave frequencies where something was being adjusted or 'peaked' in power. Obviously, the analogue display was very powerful here.

IMO the one to have was the older HP432A with the HP478A thermistor head. The HP435 supports the later (modern) thermocouple type head but the HP432A with HP478A head is the real star because it uses a closed loop system for calibration and can be very accurate and maintain this accuracy for very long periods. The HP435 needs its internal 0dBm reference because it uses an open loop cal system.

Most people either want the older HP432A or the HP power meters with a digital display and GPIB so the HP435 is kind of lost in the middle and isn't in much demand these days.

Sorry, reading this thread backwards  ; maybe this is the answer or part of the answer to the question I posted above?  Thx

[MarkL]

Where on the continuum from older not so expensive power meters (such as the one in the OP) toward something newer and perhaps more expensive might be a sweet spot of value for an RF power meter (and sensor?) if someone wanted to measure/confirm power when using a 3GHz signal generator with a 3GHz spectrum analyzer?  Any favorite HP or other models?  Any preferences on digital readouts vs. analog meters?  Thx

Over analog meters, digital meters allow you to do easy relative measurements from a reference value, and they connect via GPIB for automated tasks.  They also have store/recall for multiple setups.

I happen to like the HP 438A.  Like G0HZU points out, this is an open loop meter.  It's not quite as old as the 435B.  It has dual inputs so you can do relative measurements between two sensors.  Option 002 adds a second reference oscillator which is nice to double check your calibration.  It can be had for $80 to $100.

8480 series sensors for the 438A (and the myriad of others) can run $200 to $400.  As I said previously, if you're buying used make sure it's returnable, and also make sure it has the calibration chart on the back (Freq vs. Cal Factor).  For some reason I've seen a lot of sensors on ebay with the chart ripped off or defaced.  If you can't see it in the photos, ask.  You need this data unless you want to have the sensor re-calibrated.

A more modern-ish looking unit is the HP 437B with an LCD display.  Same price range and also uses 8480 series sensors.  It's a single channel unit.

An improvement to the 8480 series sensors is the N8480 sensors which have the calibration table built into the sensor in an EEPROM.  Although they look the same and are similarly numbered, they are incompatible with these older meters.

More modern meters have sensors with better dynamic range, and there's also USB-based power sensors.  But for just an average power reading I find these old units work fine for a reasonable price.

[Electro Fan]

Where on the continuum from older not so expensive power meters (such as the one in the OP) toward something newer and perhaps more expensive might be a sweet spot of value for an RF power meter (and sensor?) if someone wanted to measure/confirm power when using a 3GHz signal generator with a 3GHz spectrum analyzer?  Any favorite HP or other models?  Any preferences on digital readouts vs. analog meters?  Thx

Over analog meters, digital meters allow you to do easy relative measurements from a reference value, and they connect via GPIB for automated tasks.  They also have store/recall for multiple setups.

I happen to like the HP 438A.  Like G0HZU points out, this is an open loop meter.  It's not quite as old as the 435B.  It has dual inputs so you can do relative measurements between two sensors.  Option 002 adds a second reference oscillator which is nice to double check your calibration.  It can be had for $80 to $100.

8480 series sensors for the 438A (and the myriad of others) can run $200 to $400.  As I said previously, if you're buying used make sure it's returnable, and also make sure it has the calibration chart on the back (Freq vs. Cal Factor).  For some reason I've seen a lot of sensors on ebay with the chart ripped off or defaced.  If you can't see it in the photos, ask.  You need this data unless you want to have the sensor re-calibrated.

A more modern-ish looking unit is the HP 437B with an LCD display.  Same price range and also uses 8480 series sensors.  It's a single channel unit.

An improvement to the 8480 series sensors is the N8480 sensors which have the calibration table built into the sensor in an EEPROM.  Although they look the same and are similarly numbered, they are incompatible with these older meters.

More modern meters have sensors with better dynamic range, and there's also USB-based power sensors.  But for just an average power reading I find these old units work fine for a reasonable price.

MarkL - Thanks.

Here is a photo of a 8481A (works with a 437B and possibly other meters); it has the cal data on it; any thoughts on this sensor? 

Also, what's your preference on the digital readout vs. an analog meter, vs. the digital readout plus simple analog meter layout on a meter like the 437B?

Thx, EF

[G0HZU]

The analogue meter is yours for the cost of postage if you want it, I won't be reusing that part of the unit.  PM me if you're interested.

Thanks very much for the offer but, sadly, I think the postage costs would be too high to send it to the UK. But someone in your country will want this meter for the same reason as me so don't throw it away...

[G0HZU]

Here is a photo of a 8481A (works with a 437B and possibly other meters); it has the cal data on it; any thoughts on this sensor? 

It's a fairly common type where the lower frequency limit is 10MHz and it probably gives best performance (in terms of minimising uncertainty) above about 25MHz.

You can also get the HP8482A power head that goes from 100kHz to about 4GHz but these may be harder to find at a cheap price. They are both very easy to damage though.

[MarkL]

MarkL - Thanks.

Here is a photo of a 8481A (works with a 437B and possibly other meters); it has the cal data on it; any thoughts on this sensor? 

Also, what's your preference on the digital readout vs. an analog meter, vs. the digital readout plus simple analog meter layout on a meter like the 437B?

Thx, EF

The sensor looks like it's in pretty good shape, and the table is there.  I've seen some sensors that have a graph and not a table.  I prefer the table since it's easier read and type in the cal factor percentage for the frequency you're measuring.  The cal has expired, but it's very recent which is also nice.

It's shown connected to the cal output of the meter, which on HP meters is 0dBm (@ 50MHz), so the display is correct.

I have an 8481A and it's probably the sensor I use the most.  It can directly measure -30dBm to +20dBm, 10MHz to 18GHz.  If that's your desired power range I think this would be a good set.  Another good sensor to have is the 8481D which measures -70dBm to -20dBm.  The 8481D needs a 11708A calibration attenuator to connect to the meter cal output (added expense).

I've never found a need for the analog meter, but I can imagine some people like it for visual feedback while tweaking or maybe finding an intermittent.  I figured I could always connect an analog meter or a fast moving Fluke with a bar graph to the recorder output on the back if I needed something like that.

[G0HZU]

Also, what's your preference on the digital readout vs. an analog meter, vs. the digital readout plus simple analog meter layout on a meter like the 437B?

I don't think I've ever used a 437 meter but I would expect that the little analogue meter is there to tell you where the meter is (auto?) operating in terms of its amplitude ranges. A bit like a DVM operates in a number of voltage or current ranges.

The measurement uncertainty is different across each range for a typical power meter/head combo and the little analogue meter window probably tells you when you are nearing the lower edge of a range and the instrument uncertainty will be degrading.

If this happens then an advanced user might switch to manual ranging to try and keep the little analogue meter needle nearer to the upper end of its range.

[Electro Fan]

Here is a photo of a 8481A (works with a 437B and possibly other meters); it has the cal data on it; any thoughts on this sensor? 

It's a fairly common type where the lower frequency limit is 10MHz and it probably gives best performance (in terms of minimising uncertainty) above about 25MHz.

You can also get the HP8482A power head that goes from 100kHz to about 4GHz but these may be harder to find at a cheap price. They are both very easy to damage though.

The reason that the 8481A and the 8482A are easy to damage is that they are thermocouple sensors instead of say the thermistor sensor used in the 478A?

[G0HZU]

Here is a photo of a 8481A (works with a 437B and possibly other meters); it has the cal data on it; any thoughts on this sensor? 

It's a fairly common type where the lower frequency limit is 10MHz and it probably gives best performance (in terms of minimising uncertainty) above about 25MHz.

You can also get the HP8482A power head that goes from 100kHz to about 4GHz but these may be harder to find at a cheap price. They are both very easy to damage though.

The reason that the 8481A and the 8482A are easy to damage is that they are thermocouple sensors instead of say the thermistor sensor used in the 478A?

The thermistor sensor in the 478A is even more fragile     You are supposed to keep it below 30mW to avoid damage. I think the modern thermocouple sensors can handle 400mW briefly but you aren't supposed to go above 100mW. It's best to keep the input level lower than this for best accuracy and I rarely put more than 10mW into a thermocouple power head.

[G0HZU]

I think I've posted these plots up before but here's the datasheet/brochure spec for one of my sig gens in terms of accuracy at 0dBm up to 4GHz. It overlays the results for several different examples of this sig gen to show the spread. My particular sig gen had been calibrated at Trescal just before I bought it so it should offer similar performance hopefully

Then here's the result of me measuring it using my Anritsu (modern digital display and thermocouple sensor) power meter and also my 50 year old HP431C power meter using a HP478A thermistor sensor.

You can see they both agree very well but once the frequency gets bove about 2GHz the measurment uncertainty of the 478A head is easy to see in the form of ripple in the 431C plot. This is because the source impedance of the Agilent sig gen isn't an accurate 50 ohm and the Anritsu power head has much lower VSWR than the HP478A sensor and so it shows much lower ripple.

But both power meters are doing very well here IMO!

I've had the HP431C/478A for about 20 years and it's never been calibrated in all this time. But you can see how accurate this old meter still is! The HP432A is a newer version of this meter and is smaller and easier to use and has auto zeroing.

[Electro Fan]

Here is a photo of a 8481A (works with a 437B and possibly other meters); it has the cal data on it; any thoughts on this sensor? 

It's a fairly common type where the lower frequency limit is 10MHz and it probably gives best performance (in terms of minimising uncertainty) above about 25MHz.

You can also get the HP8482A power head that goes from 100kHz to about 4GHz but these may be harder to find at a cheap price. They are both very easy to damage though.

The reason that the 8481A and the 8482A are easy to damage is that they are thermocouple sensors instead of say the thermistor sensor used in the 478A?

The thermistor sensor in the 478A is even more fragile     You are supposed to keep it below 30mW to avoid damage. I think the modern thermocouple sensors can handle 400mW briefly but you aren't supposed to go above 100mW. It's best to keep the input level lower than this for best accuracy and I rarely put more than 10mW into a thermocouple power head.

Oh, other than exactly backwards, I had it right 
Thanks

[MarkL]

Just took a quick stroll through the 437B manual.

It has one really nice feature where you can store the cal factors for up to 10 sensors and then just enter frequency of interest.  You don't have to keep looking at the printed sensor cal table.

[Electro Fan]

Just took a quick stroll through the 437B manual.

It has one really nice feature where you can store the cal factors for up to 10 sensors and then just enter frequency of interest.  You don't have to keep looking at the printed sensor cal table.

That IS nice ; after getting the first sensor there would be very little friction to inhibit buying about 9 more... 

Seriously, it is a good feature.

I just quickly looked through the 438A manual - it has support for two sensors at the same time but I didn't see the feature that would let you save the settings for up to 10 sensors (but I might have missed it).

[G0HZU]

My Anritsu power meter is a similar vintage and offers the same cal factor storage feature although I'm not sure if it supports multiple sensors in memory. It probably does.

But it is much easier to just use it manually because it has a rotary control to set the cal factor and it is a breeze to adjust it like this. By comparison, it isn't that intuitive how to enter all the cal data and it would take a fair while to do and you still have to dial in the frequency once completed. But again this is done with a rotary control so it's much easier than typing the number in.

When I did the above plots I did it remotely and stored all the cal factors in the PC and the program used interpolation between points to really get the best performance from it. The HP431C doesn't have GPIB so it was logged from the analogue recorder/DMM port at the back. The cal factor vs frequency on the HP478A sensor is fairly flat so I just used one fixed cal factor across the whole 4GHz range. I think it would have been slightly better if I had corrected it for cal factor but this would still have been dominated by the ripple caused by mismatch uncertainty in the 478A sensor head

[MarkL]

I just quickly looked through the 438A manual - it has support for two sensors at the same time but I didn't see the feature that would let you save the settings for up to 10 sensors (but I might have missed it).

No, you didn't miss it.  Sadly, the 438A doesn't have the cal table feature.

It was either the cal table or the dual channels.  And the 438A also had a metal case and a readily available service manual with full schematics.  So went with the 438A.

It also has an loud fan (same as the 8116A), so if noise bothers you don't get a 438A.  In my lab it's just one of many, so I'm used to it.

[Rupunzell]

Story behind the small wiggle meter on the hp 436A, 438A and others. When the hp 435 series of thermocouple based power meters were introduced the analog meter as typical and allowed observation in rates of change in power. Observing rates of change is poor to not possible with digital read out type displays. Here is a link to the hp journal introducing the 435 series of power meters. It is a LOT more complex and cleaver than it appears. An appreciation of how it worked needs to be studied to understand just how well the folks at hp addressed this design problem and solved it in a ver enduring way that lives on to this day.
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1974-09.pdf

RF power meters are much like a "Volt-Ohm-Amp" meter in the world of RF and microwave folks. Since most RF systems are 50 ohm based, this makes a plain and basic volt or current or current meter less useful as having a fixed impedance means power is more relevant than just volts, or current as either of those units are not relative to a impedance unless the impedance is known. This is why power based on a 50 ohm system is used as a normalization for measuring power in RF and power systems.

With the introduction of the hp 435 & hp 8481, power measurements were made easier. Before this system, thermistors/thermistor mounts and all the problems associated with that system of measurement was a source of never ending niggles. The analog meter work well, it was accurate, allowed rate of change observation and was reliable, durable and accurate.

In time the folks at hp decided to update aka "improve" the 435 series of meters by offering a digital readout version known as the 436A. During instrument trials, users trying to observe rate of change (in power) discovered it was near impossible with the digital display. To address this real world instrument requirement, the folks at hp added a small wiggle meter specifically designed and intended to display rate of change.

In the home lab, the 435A/B. 436A, 438A (fan sounds like a jet taking off) all live together as the complements to each other and allow great power measurement flexibility. The power meters are common, the power sensors and cables are precious as they are compatible over a broad range of hp power meters. These can be amazingly accurate for years and years. The power sensor heads can be fragile and easily damaged if excessive power is applied. They are tolerant to this kind of abuse to a limit. Go past that limit and the power sensor is dead.
Wiggle meters have their place and offer advantages digital displays cannot. Another case of knowing how to apply what work best for a given measurement requirement.

In the passing decades since the introduction of hp's thermocouple based power sensor, came the diode based power sensor and eventually other manufactures got into this game including Gigatronis (previously Systron Donner),  Wiltron / Anritsu and others including made in China knock-offs.

 
Bernice

[G0HZU]

Wiggle meters have their place and offer advantages digital displays cannot. Another case of knowing how to apply what work best for a given measurement requirement.

My Anritsu power meter has a digital display but also has a very similar type of 'wiggle' meter fitted as the HP436/7.

The wiggle meter looks like a direct copy 

However it compares really badly with my HP431C power meter dial in terms of 'wiggle' feedback in a visual sense. i.e. you have to hunch forwards and look very closely to use it and the little analogue meter is obviously a low cost thing on my Anritsu. By comparison the big old dial on my 431C can be used at a fair distance and gives good visual feedback.
it's not surprising that these tiny cheap wiggle meters were phased out pretty quickly.

I use mine mainly to let me know when the meter is near the low edge of a range and the measurement uncertainty is degrading. I suspect that other users will do the same.
If I want to use the (digital) Anritsu meter as an analogue 'wiggle' meter I can always connect the analogue recorder output to an analogue voltmeter. eg something like an AVO 8 and get a nice big wiggle meter. The AVO will probably load the output slightly (compared to a DMM) but I guess this doesn't matter for a wiggle meter. The alternative would be to log it over GPIB and have a peaking meter display on a PC/Laptop.

But usually I would rather work with a spectrum analyser with a 20dB attenuator selected if peaking something up

[Rupunzell]

That plastic wonder of what appears to be a wiggle meter has got to be one of the lowest cost wigglers they could have possibly used. It was all they had room for in the 436A as oops correction. That is why it looks less than ideal on the front panel layout.

Indeed, the 413/435 series have proper hp wiggle meters of proper size, mirrored scales and needle movement dynamics. These features are not often appreciated today as digital readout ease has replaced learning and understanding the advantages analog wigglers offer over a digital readout.

I do believe reading an analog meter properly is a learned thing that comes with time and practice and experience. Digits are direct reading like spoon feeding.

The set of power meters here are good to fractions of a db which is really good for a power meter. Still using an SA delivers a LOT more information about the DUT overall.


Bernice

My Anritsu power meter has a digital display but also has a very similar type of 'wiggle' meter fitted as the HP436/7.

The wiggle meter looks like a direct copy 

However it compares really badly with my HP431C power meter dial in terms of 'wiggle' feedback in a visual sense. i.e. you have to hunch forwards and look very closely to use it and the little analogue meter is obviously a low cost thing on my Anritsu. By comparison the big old dial on my 431C can be used at a fair distance and gives good visual feedback.
it's not surprising that these tiny cheap wiggle meters were phased out pretty quickly.

I use mine mainly to let me know when the meter is near the low edge of a range and the measurement uncertainty is degrading. I suspect that other users will do the same.
If I want to use the (digital) Anritsu meter as an analogue 'wiggle' meter I can always connect the analogue recorder output to an analogue voltmeter. eg something like an AVO 8 and get a nice big wiggle meter. The AVO will probably load the output slightly (compared to a DMM) but I guess this doesn't matter for a wiggle meter. The alternative would be to log it over GPIB and have a peaking meter display on a PC/Laptop.

But usually I would rather work with a spectrum analyser with a 20dB attenuator selected if peaking something up

[Electro Fan]

In watching this thread evolve it seems that there is a bit of a consensus developing that a Spectrum Analyzer not only delivers a lot more information than a Power Meter but that it might be the preferred tool for power measurements in general.  Or did I misinterpret that?

Some reasons to use a Power Meter:

1) It's less expensive than a SA and a SA might not be available
2) The less expensive PM might be a first line tool to protect a more valuable SA (in case something should glitch with a power reading)
3) The PM is smaller and maybe more versatile in not only size but with especially with some meters that can support 2 sensors at the same time
4) The PM could help confirm measurements from other equipment (SA, Sig Gen, etc.)
5) ? I'm out of ideas

So, is it possible that investing in a SA would largely preclude the need for a Power Meter?  (thereby freeing up funds toward a SA, or a better SA?)  Or is a Power Meter a basic tool like a screwdriver, wire stripper, etc.?

Thanks for all the insights.

[MarkL]

The SA is going to provide infinitely more information.  There's no question of an SA over a power meter if you only get to buy one of them.

The power meter is only going to tell you the average power of *everything* coming in, including all the harmonics and other distortions of a signal.

The primary reason I have a power meter is to calibrate my SAs.

[dom0]

The power meter is only going to tell you the average power of *everything* coming in, including all the harmonics and other distortions of a signal.

Doesn't that depend on the type of detector? Or do they all use thermal detectors?

[MarkL]

The power meter is only going to tell you the average power of *everything* coming in, including all the harmonics and other distortions of a signal.

Doesn't that depend on the type of detector? Or do they all use thermal detectors?

There are peak sensors too, but these legacy ones that we're talking about are only average power sensing.

Some sensors are diode based, where you can get much higher sensitivity.  Here's probably more than anyone would want to know on various sensor technology (part 2 of a 4 part series):

  (2) Sensors and Instrumentation: http://literature.cdn.keysight.com/litweb/pdf/5988-9214EN.pdf

The other parts are:

  (1) Intro and Standards:  http://literature.cdn.keysight.com/litweb/pdf/5988-9213EN.pdf

  (3) Measurement Uncertainty:  http://literature.cdn.keysight.com/litweb/pdf/5988-9215EN.pdf

  (4) Keysight Offerings:  http://literature.cdn.keysight.com/litweb/pdf/5988-9216EN.pdf


(Also, I'm not trying to be overly biased towards HP/Agilent/Keysight.  Other companies certainly sell power meters, but this is the only gear I'm familiar with.)

[G0HZU]

I use both a classic lab power meter and a spectrum  analyser to measure RF power. But usually the power meter is king if you want to measure power of a (dominant) signal with minimal measurement uncertainty.
However the latest generations of lab grade spectrum/signal analyser offer quite low uncertainty for power measurement.

Traditionally, the power meter was also king when measuring the total power of broadband noise but modern spectrum analysers (and some scopes) can do this remarkably well nowadays.
But you do need a sensor port with very low VSWR to get the uncertainty low.

The sensors for my Anritsu power meter maintain a typical port VSWR of 1.05:1 across a huge bandwidth and have minimal variation in cal factor across a huge BW. This makes them very useful for measuring broadband noise accurately.

Obviously, a spectrum analyser can see much smaller signals than a classic power meter so it has an edge here and you can also ask some of them to measure average power across a defined bandwidth/window or to compute average noise density across such a window. This can be very useful and a power meter can't compete here.

But a typical spectrum analyser made in the last 30 years will have so much overall measurement uncertainty that it can't be relied on to be within even +/-1dB. Some (many) will be much, much worse than this. Only the very best of the modern generation of $$$ signal/spectrum analysers can improve on this performance.

With careful management of overall measurement uncertainty a decent lab power meter should often get within +/- 0.15dB if used where its sensor VSWR is low and where the sensor range is at its most linear/accurate. But the overall uncertainty (worst case analysis) is much worse than this. Hence the need to try and manage/control uncertainty as much as possible.

[Rupunzell]

hp 438A example. Quick test using the cal output and with an attenuator spec'ed at 10db with label curve noted at 9.7 db at it's lowest range ( 0 Ghz) which is good enough for the cal signal of 50Mhz set to relative zero db on the 438A. Display on the 438A reads -9.63db, or if all is correct a difference of 0.07db.

438A, 0db ref set, cal factor 98% (from power sensor).



438A, display reads -9.63db. Attenuator is noted -9.70db at (0 Ghz ).



These power meters are accurate and does well at what they are designed to do when used properly.



Bernice

But usually the power meter is king if you want to measure power of a (dominant) signal with minimal measurement uncertainty.

Obviously, a spectrum analyser can see much smaller signals than a classic power meter so it has an edge here and you can also ask some of them to measure average power across a defined bandwidth/window or to compute average noise density across such a window. This can be very useful and a power meter can't compete here.

But a typical spectrum analyser made in the last 30 years will have so much overall measurement uncertainty that it can't be relied on to be within even +/-1dB. Some (many) will be much, much worse than this. Only the very best of the modern generation of $$$ signal/spectrum analysers can improve on this performance.

With careful management of overall measurement uncertainty a decent lab power meter should often get within +/- 0.15dB if used where its sensor VSWR is low and where the sensor range is at its most linear/accurate. But the overall uncertainty (worst case analysis) is much worse than this. Hence the need to try and manage/control uncertainty as much as possible.

[N8AUM]

Hello all,
just picked up my 1st power meter, HP-435B off of fleabay. Any ways, what are some of the symptoms of a bad sensor ? I cannot zero it and when I apply a small signal it reads more than 30db greater than it should be. It came with the 8484 sensor.

N8AUM

 

[MarkL]

Hello all,
just picked up my 1st power meter, HP-435B off of fleabay. Any ways, what are some of the symptoms of a bad sensor ? I cannot zero it and when I apply a small signal it reads more than 30db greater than it should be. It came with the 8484 sensor.

I'm assuming that would be a 8484A?

The 8484A sensor is a high-sensitivity diode based sensor, so by design it has a 30dB offset once calibrated.

Did you try to calibrate it using the 30db calibration attenuator (part #11708A)?  It's similar to the one Bernice shows plugged into his 438A a couple of posts back.

If you already did this, does the sensor calibrate and it just won't zero?

My reading of the schematic is that it could be either in the sensor or the meter.  There are some troubleshooting steps in both the 435B and the 8484A manuals, if you want to get into it.

But if you start troubleshooting, many sellers won't consider a return once the covers are off.  And if it's the sensor, you're going to be stuck with an unobtainium repair.


I don't have the 435B or that sensor, so maybe someone can comment who has direct knowledge if there's any quirks with that meter/sensor pair.

[N8AUM]

Thanks for the reply MarkL. I just noticed I made an error when I described the symtom. With a 30db attenuator on the cal output I can zero the meter but only if I set the meter to +10db and not 0db like it says to do in the manual.

[MarkL]

When you say "zero" do you mean while applying power input to the sensor?  To be clear on terminology, it's a two-point procedure. 

The first step is "zeroing" where no power is applied to the sensor input and the "Zero" switch is pushed on the front.  This automatically balances out any residual offset in the amplifier circuits.

The second is attaching the sensor to the front cal output and adjusting for a 0dBm (or 1mW) reading with Cal Adj.  In your case with the 8484A you need the 30dBm attenuator in there and you would still adjust for 0dBm (although the actual sensor input would be -30dBm).  I think on this meter the Cal Factor is always active, so you might also have to set this to the sensor's cal factor for 50MHz (usually 100%, unless printed otherwise on the sensor).

I think you're saying the second step is not behaving quite right?

[N8AUM]

I was referring to calibrating using the 1mW internal reference. Anyways, I just fingered it out, it has the wrong scale for this sensor    !
Damn, cant believe it took me this long to figure it out !   

[MarkL]

Great - glad you figured it out!

The manual mentions something about swapping range scale rings on page 2-1.  I guess that was it?

[N8AUM]

Great - glad you figured it out!

The manual mentions something about swapping range scale rings on page 2-1.  I guess that was it?

Wish it would have clicked when I read it but guess it didnt. Had to take a picture of the ring I need and paste it to the original ring that came with, not the prettiest but it will do.

[MarkL]

The manual says there should be two scale rings stacked behind the scale knob (with three different scales - I guess one is double-sided).  The knob unscrews so you can put the scale you want in the front.

Or maybe the ring you need is gone?  Such is the luck with used equipment.

[N8AUM]

The manual says there should be two scale rings stacked behind the scale knob (with three different scales - I guess one is double-sided).  The knob unscrews so you can put the scale you want in the front.

Or maybe the ring you need is gone?  Such is the luck with used equipment.

It just had one ring. I just purchased another 435A, hopefully it will have the ring I need.