High isolation LNA for the HF band direct conversion receiver

[iMo]

The FST3253 mixer has high IP3 thus it would require something more elaborate in front of it (than a single jfet), imho..
PS: Norton feedback preamp?

[KE5FX]

Backward isolation is not really a problem at 130 dB?

Ah, I thought you were complaining that it wasn't good enough (presumably at 100 MHz.)

80 dB at 100 MHz isn't bad at all, depending on what you need it for.  You won't have injection-locking problems at that level, certainly.

[UR5FFR]

Hi

The FST3253 mixer has high IP3 thus it would require something more elaborate in front of it (than a single jfet), imho..

You right - for FST/FSA based mixer preamplifier must be have high IP3. By example 4*J310 with total current 80-100mA or BFG541/BFG591 with current 70-100mA.

[iMo]

Norton-feedback one? Not sure on isolation, but for HF you do not need much, imho..
Some measurements:
https://www.kn5l.net/rf-amp/norton/
https://www.okdxf.eu/lankford/Noiseless%20Feedback%20Norton%20Amplifiers.pdf

[UR5FFR]

Norton-feedback have poor isolation. It input impedance depended from the load

[Gerhard_dk4xp]

LTspice sim added for completeness.

An RF preamp is probably not needed on short wave for sensitivity, more to reduce osc emissions.
Much more important is proper termination of the RF and IF ports of the mixer.
Everything that is reflected into the mixer undergoes a new cycle of mixing,
creating higher order IMD in the process.
V3 does nothing interesting; it just prevents LTspice to do a divide by zero or sth. like that.
60 Ohms at room temp. delivers 1 nV/rt Hz thermal noise. Unimportant in the time domain.

[szszjdb]

Hi,dk4xp,
Thanks a lot!
It is great！ That is the multi-stage cascode circuit. How about the current drain and the output impedence?  The common base seems a little high output impedence.

Best Regards,
James

[Gerhard_dk4xp]

The collector current varies between 5 and 125 mA or so, as you can see in the red trace
of the upper plot pane. The idle Icc is about 65 mA. This is for 13 dBm max. output.
If you accept 10 dBm or even less you can reduce that by increasing R8. Same when
using a 4:1 output transformer. But the transformers I had in the drawer gave weird S22.
For 13 dBm, the BFQ19S with its 120 mA Ic abs max is just so enough. BFQ790 should be comfy.

Here the transistors behave as an ideal current source and the 50 Ohm resistor dictates
Zout. the 50 Ohm resistor at the right is the simulated load.

Q2 is a voltage-to-current converter; The emitter resistor is the scale factor.
The PNP only adjusts the operating point.

This was intended as one channel of a distribution amplifier. You can parallel some
inputs that are medium impedance and adjust Zin to 50 Ohm with a resistor.

[KE5FX]

The series combination of D1 and D2 is interesting.   Those low-voltage (true) Zener diodes are next to worthless in my experience; do you actually find that they are doing anything at all?

Seems like a 6.1 volt Zener (i.e., avalanche) diode would do better, but I guess the reasoning is that it would be noisier.  Maybe a couple of blue LEDs would work?

[Gerhard_dk4xp]

The true Zeners work. Yes, they are softer. But it is not important if it is 100 mV more or less.
Maybe it would work without the diodes but I wanted less unknowns / easier changes
when debugging the bias system.

One of the pictures is the noise of  Zener diodes. One can easily see the strong rise
of the noise vs. rise of Vz. Between 2V7 and 6V are worlds, in the same series.

Diodes are tested with a 1k or 2k wire resistor from 10 NiCd cells.
The NiCds add no visible noise.

The steep rise with 1/f**3 on the left side of the plots goes on the measuring
amplifier resp. its input coupling capacitor.  6 years ago, that was 100 uF foil,
not enough by far. The short & 60 Ohms are on the amplifier side of the cap
and show what the amplifier can do really with a low impedance DUT.
A 4700 uF wet slug tantal healed that. The input stage of the preamp consists of
20 ADA4898 op amps (10 pairs) in par and averaged into an inverter.
Noise in the flat part is abt. 220 pV/rt Hz . The 0 dB line is 1 nV/rt HZ.

I now have an amplifier based on 16 CPH3910 FETs that is nearly in the same
league but with less noise current and a reasonable input cap. I must find
the time time for the write-up. The smaller noise current also opens the
way to cross correlation with the Agilent 89441A. The effect of the noise current
is common mode in the DUT and would not average away.

Among LEDs, blue ones are bad. King of LEDs is HP/Agilent/Avago
HLMP-6000 if you want low noise. Otherwise, it's quite a dim bulb.
There is also a LT3042 for comparison.

For completeness, there are also some regulators.
The LT3042 is standing out.

The LM329 is from Digikey, marked NS, years after TI bought NS.
It does not look like a Zener, more like a bad bandgap.

[KE5FX]

Good stuff.  I have a bunch of CPH3910s in the drawer, as well as an 89441A that could sure use a quiet preamp, so will keep an eye out for more details on that. 

[Gerhard_dk4xp]

The traces are taken with the 89441A. Since it cannot make nice pictures over
several decades, I have written a program to control it via thin Ethernet / GPIB and
make measurements over ~ 7 decades. Then the results are plotted with Gnuplot.
I never got GPIB working completely with that yellow USB/GPIB dongle.

I found out that I have to do a seek if I switch the direction of the socket interface,
even if not wanted, and newer Linux kernels got more strict about that from release
to release.  After that there was no more point in using GPIB.

[szszjdb]

Dear All,

I have another question about the LNA.
How to determine  the proper MAX 1 db compression power output for the LNA in the HF band receiver , given that the quadrature detector and the following ADC saturation point at -5 dBm ?
The bigger should have better OIP3 performance but more current drain. It should also  consider about the in band and out band interference ,especially the AM BC signal. The famous KX3 use only a common emitter amplifier with the P1dB for about +5 dBm and 10 mA bias. That is enough?

Best Regards,
James

[iMo]

It is not only matter of IP3 but also of input band pass filtering.

[A.Z.]

It should also  consider about the in band and out band interference ,especially the AM BC signal.

James, I already wrote it and I'll repeat it again; don't put a wideband preamp with high gain directly at the antenna input; put a (set of) bandpass filter(s) at the antenna and connect your preamp after the filter, this way the preamp will only work on the portion of frequencies of interest, also ensure to properly adapt the input and output impedance of the bandpass so that it will work properly, a simple trick to do that (but there are a number of other ways), assuming the antenna input has an impedance of 50 or 75 Ohms, could be adding a simple common gate stage (again ) configured for LOW gain at the antenna input, the output of this stage will go to the bandbass and the output of the bandpass to the preamp, the preamp should present an input impedance similar to the one at the output of the common gate and the bandpass should be designed to work at such impedance

[szszjdb]

Hi, imo and  A.Z.,
Many Thanks!

Yes there have a bandpass before the common gate stage and the switchable preamp in my design. In the normal condition of lower HF band, there is no need for the preamp, and the IP3 performance is determined by the low gain common gate and the QSD itself. That should be no problem.  But in the higher aband like 15m to 6m , there will often need that preamp.  The out band signal I mentioned means the  band nearby. Due to the wide span of the bandpass  filter , e.g. the 20m's is from 8-16Mhz  , the 30m signal will come with the 20m band when preamp is on. So I am considering the proper P1dB or the OIP3 performance to just meet the requirement of the following QSD stage.

Best Regards,
James

[A.Z.]

wide span of the bandpass  filter , e.g. the 20m's is from 8-16Mhz

your bandpass is too wide, as I see it (hope others will correct me if I'm wrong), you have two possible solutions; one is use narrower filters, one for each band of interest, another (but may be mixed with the first one) is using tunable filters so that a single filter may cover a given range but still have decent bandwidth to cutoff the adjacent bands

[Gerhard_dk4xp]

Back when I found shortwaves interesting, I built a filter that cut
the 80m band into 4 ranges.
5 Siemens K1 pot cores with Q> 400, reed relays and mechanical trimmers.
Sorry, can't find it for a photo.   :-)

[UR5FFR]

Hi

I can add some important thing about BPF for direct conversion receiver. You need suppress 3F and 5F frequency as much as you can. Key mixer converts on odd LO harmonics. On 3F frequency it have -10dBm conversion gain! This is mirror channel for DC receiver.
So when you choose BPF check it in simulator (by example RFSimm99)

[A.Z.]

Back when I found shortwaves interesting, I built a filter that cut
the 80m band into 4 ranges.
5 Siemens K1 pot cores with Q> 400, reed relays and mechanical trimmers.
Sorry, can't find it for a photo.   :-)

I think that it was a pretty interesting thing

Getting back on topic, if you look at my earlier post about that "Steber" tuned amplifier, such a thing may help solving the issue, what I mean is; let's say that the OP has a 20m band bandpass filter with a range going from 8 to 16 MHz, now, if we add the simple varactor tuned (or a better one btw) circuit in front of (or right after) the bandpass and adjust it following the main tuning...

[szszjdb]

Hi, A.Z., dk4xp and UR5FFR,
Many Thanks!

The existing 30-20m bandpass have a 3dB bandwidth from 9.5 to 16.9Mhz and -40dB at 30Mhz and -60dB at 42Mhz. I will fine tune the filter and there might have some trade off as I am designing a portable QRP unit.  As running by the battery, the current drain is critical.
Then how about the P1dB or OIP3 requirement for the following preamp? Does a P1dB +5 dBm with 15-20dB gain preamp fulfill to the QSD with -5dBm saturation point?

Best Regards & 73,
James

[A.Z.]

forgive me, but I can't see how a rx bandpass filter would have an impact on battery drain

[edit]

a set of bandpass filters, one for each ham band won't require too much components (L/C) and will help avoiding out of band issues

[szszjdb]

Hi, A.Z.,

Sorry, I mean the preamp. The higher OIP3 the higher current drain and the Vcc. I just want to make sure the bias point of the preamp is just fulfill to the system requirement. What should be the banlance point?
I am considring refining the bandpass filter in the very front end.

Best Regards,
James

[Gerhard_dk4xp]

Some pointers:

Really about noise blankers, but that's the same area in the receiver:
<    https://dk4sx.darc.de/eigenbau_rx.htm     >
<    http://www.oe3hkl.com/rx-homemade/rx-mit-h-mode-mixer-ip3-45dbm/stoeraustaster.html    >
<    https://worldradiohistory.com/Archive-DX/Ham%20Radio/80s/Ham-Radio-198006.pdf    >


<    https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1994/05/page29/index.html  >
<    https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1994/06/page27/index.html    >
<    https://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/qst/1994/07/page42/index.html   >

There is a lot more on Rob's web IIRC.

S9 is defined as 50uV in the 1/3 Rohde articles.  0 dBm is 635 mV
S9 = 50 uV is -73 dBm for comparison.

73, Gerhard

[szszjdb]

Hi, dk4xp,

Many Thanks!
Your information help much!

It seems the preamp  P1dB should have 3 dB higher than the QSD and following . As the saturation point of QSD is about -5 dBm, the 0 dBm P1dB  of preamp should be enough for my system. Is that right?

Best Regards & 73,
James