AOR-8200 MkIII max input RF level + DC bias

[OatmealEater]

Hi everyone,

I'm new here. Electronics hobbyist for decades but not really into RF. Been
using my AOR for a few years mainly to listen to emergency broadcasts, and I'd
like to get into the GHz-range circuitry.

So I got myself a 3GHz synthesizer (a PLL VCO dev board from Analog) but the
only GHz-capable instrument I have is the AOR itself. I want to use it as a
primitive spectrum analyzer to check the center frequency and harmonics produced
by the synthesizer.

Question: How can I connect the synthesizer to the AOR without (a) damaging the
AOR and (b) damaging the synthesizer and (c) creating unwanted transmissions.

I have a 10 dB attenuator and the board can attenuate its output to -4 dBm.
but I think that's not enough.

I found some info on the AOR S-meter levels here:
http://www.thiecom.de/ftp/aor/ar8200/information/ar8200bul.pdf

From their table on page 10 we see that the S-meter display saturates at -66 dBm
but they provide data up to 10 dBm, so -14 dBm sounds safe.... except that:

I have a schematic for the AOR 8200 (not MkII or MkIII) and the input consists
of a 1SS226 dual diode that acts as a limiter followed by a .1 nF cap.

At -14 dBm the power is 40 mW and the voltages should be 1.4 V in the 50 ohm
line, which will definitely be clipped by the 1SS226, causing reflections that
may or may not damage the synthesizer output stage. Worst case the diodes will
conduct more than 100 mA possibly exceeding their 150 mW limit (although they
will be conducting half a cycle each).

In addition there is the possibility that there is DC bias on the synthesizer
output.

I think what I need to do is:

1) Get a stronger attenuator to bring the signal below to -66 dBm where the
S-meter saturates. So I should get a 70 dB attenuator.

2) Get a DC block to eliminate any DC bias.

Also, I'm using RG58/U cable, but I'm not sure of its quality.

Any thoughts or words of advice will be greatly appreciated.

 

[prcguy]

Sounds like you've done some homework and nobody would ever know your were not really into RF.

The levels available from your synthesizer will not harm the AOR but I would attenuate the fundamental frequency down to maybe -40dBm or a level where the receiver is not likely to produce Intermod and skew your search for harmonics from the synth.

I don't have a diagram for the AOR but I suspect the circuit around the input limiting diodes is such that they will not conduct at any normal level encountered by the receiver and I've measured 10dBm aggregate power from a simple Radio Shack Discone, so they probably conduct at a point higher than that. I say this because when these diodes conduct they will produce a mess of harmonics that would ruin reception and I've had my AR8200 connected to outside antennas in very high RF level areas with no indication of this problem.

If you have any attenuation in line it will also isolate the synthesizer from mismatch and damage if the diodes conduct. For example a 10dB attenuator will provide 20dB return loss to the synth even if one end is open or shorted. If the Synthesizer outputs DC then you will need a DC block.

Your own recommendation of 70dB attenuation will leave you with about 60dB measurement range assuming the receiver can hear down to maybe -126dBm. I think you can raise that 20dB by using a 50dB attenuator and still make useful measurements. If you call using a receiver with an uncalibrated meter and some internal birdies a measurement.
prcguy

Hi everyone,

I'm new here. Electronics hobbyist for decades but not really into RF. Been
using my AOR for a few years mainly to listen to emergency broadcasts, and I'd
like to get into the GHz-range circuitry.

So I got myself a 3GHz synthesizer (a PLL VCO dev board from Analog) but the
only GHz-capable instrument I have is the AOR itself. I want to use it as a
primitive spectrum analyzer to check the center frequency and harmonics produced
by the synthesizer.

Question: How can I connect the synthesizer to the AOR without (a) damaging the
AOR and (b) damaging the synthesizer and (c) creating unwanted transmissions.

I have a 10 dB attenuator and the board can attenuate its output to -4 dBm.
but I think that's not enough.

I found some info on the AOR S-meter levels here:
http://www.thiecom.de/ftp/aor/ar8200/information/ar8200bul.pdf

From their table on page 10 we see that the S-meter display saturates at -66 dBm
but they provide data up to 10 dBm, so -14 dBm sounds safe.... except that:

I have a schematic for the AOR 8200 (not MkII or MkIII) and the input consists
of a 1SS226 dual diode that acts as a limiter followed by a .1 nF cap.

At -14 dBm the power is 40 mW and the voltages should be 1.4 V in the 50 ohm
line, which will definitely be clipped by the 1SS226, causing reflections that
may or may not damage the synthesizer output stage. Worst case the diodes will
conduct more than 100 mA possibly exceeding their 150 mW limit (although they
will be conducting half a cycle each).

In addition there is the possibility that there is DC bias on the synthesizer
output.

I think what I need to do is:

1) Get a stronger attenuator to bring the signal below to -66 dBm where the
S-meter saturates. So I should get a 70 dB attenuator.

2) Get a DC block to eliminate any DC bias.

Also, I'm using RG58/U cable, but I'm not sure of its quality.

Any thoughts or words of advice will be greatly appreciated.

 

[OatmealEater]

Sounds like you've done some homework and nobody would ever know your were not really into RF.

...

If you call using a receiver with an uncalibrated meter and some internal birdies a measurement.

Thanks for the reply!

Of course using the AOR like this is nowhere near anything that could be called a measurement.

So I got a bunch of inline SMA attenuators, but no matter how many I stringed together (I went up to 80 dB using 20 + 20 + 30 + 10 dB attenuators), it saturates the S-level on my AOR. So I think I have a serious leakage problem.

I had a similar sensitivity issue with a Crystek RF power detector (with an op-amp amplifier): there is a very important hand effect.

I think the problem is that the RG-58/U I'm using is crappy. I sliced it open and (a) it doesn't have foil shield and (b) the braid has like 50% coverage.

I think I'll have to shop for some proper coax.