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EIP 545B Frenquency counter
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douardda:
Hi everyone,

I'm still fighting against this sensitivity problem, and I have hard time understanding how the so called "Video Amplifier" (should) works.

In fact, I'm not sure I do understand the kind of signal and impedance I should expect from the YIG filter assembly. This unit do combine the filter itself (which is known to be partly broken in my unit, as shown earlier), but I'd like to understand a bit better the mixer part.

The mixer looks like the last picture from my previous post.

As far as I understand dit, it consists in two parts:

- the LO path, where there are filtering made of capacitors and inductances (the wire between capacitors) before entering the ceramic circuit, on which lies several stripline-like RF filters ending with a (directional?) coupler to the output signal from the YIG filter itself
- a mixer path, after this coupler, consisting in a few components mounted on the ceramic substract.

My hypothesis is that among these components is a mixer diode (my guess is that this is the first one after the coupler, followed by an LC filter), forming a single diode balanced mixer. Otherwise, I don't understand how this assembly can implement a mixer (since a non-linear device like a diode is required to build a mixer). 

Are these assumptions looks right? Any RF guru around there to explain a bit more this mixer/coupler device?

David


 
HighPrecision:
The programmable VCO signal (370-500MHz) need to be raised in power to have sufficient level to drive the SRD (Step-Recovery Diode), to obtain a wide comb of carriers (multiplies of VCO carrier), after this there is another circuit with the microwave mixer.

In some occasions, the old variant of A203 MW converter (2010137-xx) may have problems with two trimmer capacitors around the  VCO power amplifier, turning these usually resumes SRD RF drive power level and consequently the sensitivity of counter.
douardda:

--- Quote from: HighPrecision on February 20, 2018, 05:03:21 pm ---The programmable VCO signal (370-500MHz) need to be raised in power to have sufficient level to drive the SRD (Step-Recovery Diode), to obtain a wide comb of carriers (multiplies of VCO carrier), after this there is another circuit with the microwave mixer.

--- End quote ---

Where is this SRD located? on the A201-A (VCO) board or in the A203 YIG assembly itself (on the ceramic board embedded in the YIG filter assembly)?

BTW, on my unit, the big black resistor near the output 2SC2283 power transistor, can turn very hot, around 150°C; not sure it is expected or a sign of leakage somewhere. The resistor does not seem to suffer, however, from this (it looks physically and electrically OK).

Also, am I correct assuming this ceramic board (the output stage, after the coupler from A201-A, and before entering the A201-B amplifier/detector board for which I did reverse engineer the schematics) includes at least one mixer diode?


--- Quote from: HighPrecision on February 20, 2018, 05:03:21 pm ---In some occasions, the old variant of A203 MW converter (2010137-xx) may have problems with two trimmer capacitors around the  VCO power amplifier, turning these usually resumes SRD RF drive power level and consequently the sensitivity of counter.

--- End quote ---

Humm I'm not sure I'm ready to tweak these, not without a better understanding of how this whole unit works and how to probe it when doing adjustments...

HighPrecision:

--- Quote from: douardda on February 25, 2018, 03:43:19 pm ---Where is this SRD located? on the A201-A (VCO) board or in the A203 YIG assembly itself (on the ceramic board embedded in the YIG filter assembly)?

--- End quote ---
The SRD is obviuosly part of the microwave circuit, I assume is located on LO side of ceramic microcircuit at opposite side of directional coupler, I barely see, on your photo, a complex microwave circuit with inductances, stubs, capacitors, the SRD diode is located here.

Coupling side of the coupler receive the comb signal from LO and is terminated at ground side with a resistor, probably 50 ohm and a laser trimmed capacitor.

LO signal is directionally injected to main side of coupler toward the mixer diode, also on this main side of coupler pass the YIG filtered input signal to mixer.

Just after the mixer there are two small wires for IF output signal (127MHz).


--- Quote ---BTW, on my unit, the big black resistor near the output 2SC2283 power transistor, can turn very hot, around 150°C; not sure it is expected or a sign of leakage somewhere. The resistor does not seem to suffer, however, from this (it looks physically and electrically OK).

--- End quote ---

This resistor is connected  in series with a inductance to the RF power transistor collector terminal to feed supply current from the +12V power rail, heating of resistor is NOT normal at 150 degrees level, measure the voltage drop on resistor and the resistance value, it should be 10 ohm.

Try to short the base terminal of 2SC2283 to ground and verify if the voltage drop on resistor goes to zero volts, shorting base to ground  removes BIAS voltage from resistive divider, if the voltage drop on resistor remains high the RF transistor is probably damaged.
douardda:
Since I have very little hope in the possibility to really fix my YIG filter, I've been looking for a replacement unit (mainly on eb), and found a cheap one a couple of weeks ago, which I have received last week. It was a bit of a gamble since this (hopefully) replacement A203 is not the exact same part number as mine, but I paid 50$ delivered (and shipment was more expensive than the unit itself), so I took the risk. The unit is clean and in pretty condition:


As I said, it is not the same part number as the one I have (2010241-08 vs. 2010241-09 in my unit), however, this replacement unit seems to work fine, and now, the sensitivity is way better than before:


Note that this later picture has been taken without doing any king of calibration of the YIG fiter...

But, as I am a bit stubborn, I still want to investigate more what's wrong with my A203 unit, and thus better understand how it works.   


--- Quote from: HighPrecision on February 27, 2018, 03:09:56 pm ---The SRD is obviuosly part of the microwave circuit, I assume is located on LO side of ceramic microcircuit at opposite side of directional coupler, I barely see, on your photo, a complex microwave circuit with inductances, stubs, capacitors, the SRD diode is located here.

--- End quote ---

Nothing is obvious for me :-) I find hard to recognize the components on the ceramic microcircuit, so I'm sure of nothing.

For the pleasure, here are closer views of the ceramic microcircuit (between the VCO output and the directional coupler):







--- Quote from: HighPrecision on February 27, 2018, 03:09:56 pm ---Coupling side of the coupler receive the comb signal from LO and is terminated at ground side with a resistor, probably 50 ohm and a laser trimmed capacitor.
LO signal is directionally injected to main side of coupler toward the mixer diode, also on this main side of coupler pass the YIG filtered input signal to mixer.
Just after the mixer there are two small wires for IF output signal (127MHz).

--- End quote ---

Yes all these makes much more sense for me now. I may try to draw a schematic of this microcircuit, just for the sake of completeness...


--- Quote from: HighPrecision on February 27, 2018, 03:09:56 pm ---This resistor is connected  in series with a inductance to the RF power transistor collector terminal to feed supply current from the +12V power rail, heating of resistor is NOT normal at 150 degrees level, measure the voltage drop on resistor and the resistance value, it should be 10 ohm.

Try to short the base terminal of 2SC2283 to ground and verify if the voltage drop on resistor goes to zero volts, shorting base to ground  removes BIAS voltage from resistive divider, if the voltage drop on resistor remains high the RF transistor is probably damaged.

--- End quote ---

Yes it is a 10 ohm resistor. I'll measure the voltage across it tomorrow (or so), playing with the amp power "switch" (via I/O at address 0x1843), and I'll tweak the base term of the 2SC2283 also, and check again the max temperature it reaches (I did measure with my poor's man thermal camera, not very accurate, but very handy when fixing PCBs nonetheless).

And I still have a few tasks to do:

- calibrate the YIG filter (but I can only reach 3GHz with my E4432B),

- calibrate the power meter (which will be way easier now that I have a decent (Gigatronic) power meter with a probe that goes up to 20GHz,

- finish fixing the firmware for the digits resolution default setting (as well as the 'Offset' led indicator that remains on even if I did fix the actual frequency offset value at startup).

David
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