Spectrum analyzer as a pan adapter?

[KC2TAU]

For fun today I tried to see if I could use an HP 8558B plug in as a pan adapter and, no matter what I did, I was unable to find the IF. I was able to find the LO just fine but no output from the IF could be found. I was advised that the 8558B might not be sensitive enough by itself to be able to pick up the low level signal exiting from the radio. Thoughts?

[KA0HCP]

It could be loading down the signal.  Did you try isolating it with a capacitor between the pick off point and the probe?   

[KC2TAU]

All of the radios I had attempted this on had external jacks on their rear panels for this very purpose.

[WQ9E]

Even with a dedicated jack, your receiver may not be intended for the 50 ohm input impedance of the spectrum analyzer.  Check your receiver manual to see what it says about load impedance for this pickoff point.

I am not familiar with the HP-8558B analyzer but my Tektronix 7L13 (plug in for the 7000 series scope mainframe) and my older 1L10 and 1L20 (for the tube type 500 series mainframes) will provide a display of area broadcast stations with a few feet of wire feeding the analyzer input and both will easily pick up and display WWV directly with a short antenna feeding the analyzer input.  Both work fine as a pan adapter.  Sensitivity is in the low microvolt range for the 1L10 and greater for the 7L13.  I would be very surprised if the HP cannot display IF level signals.

[WA1GFZ]

Take a FET like a J310 and set it up as a source follower. The spectrum analyzer is a 50 ohm input. Most RX circuits are high Z. Don't put any DC on the spectrum analyzer as it could blow the mixer. Sweeping spectrum analyzers do not do as well as an FFT analyzer. A softrock on your IF wil be a much better display.

[KC2TAU]

What makes it less suited for such purposes? The IF on the radio I'll be using for this is 70mhz and from the reading I've done I have a feeling this may cause an issue.

[gerry_w1id]

You're facing two issues: the sa is loading down the signal and the signal level is too low. You're trying to view microvolt signals where the instrument measures millivolts. Even if you added a high impedance buffer at the signal source, there wouldn't be enough signal to see.

[Mike/W8BAC]

It depends on the optional front end the 8558B has. Option 001 has a sensitivity of -110 dBm or .7 micro volt. Option 002 has a sensitivity of -63 dBmV, approximately 1.0 micro volt. Check the impedance of your front end as well. Many surplus HP 8558B SA's went to AT&T with 75 ohm front ends.

If the 70 MHz IF output is between .7 and 1 micro volt you should see it on the SA. The tip on capacitive coupling 

[KC2TAU]

If that doesn't work I've heard of people downconverting their IF to something in the audio range so that it can run into the line of a PC sound card. An example I found is located here: http://mysite.verizon.net/sdp2/id12.html

Would it be possible to modify this to downconvert from 70mhz?

[KM1H]

It depends on the optional front end the 8558B has. Option 001 has a sensitivity of -110 dBm or .7 micro volt. Option 002 has a sensitivity of -63 dBmV, approximately 1.0 micro volt.

Thats pretty deaf but still fine even at the antenna for strong signals.

Considering that 70MHz is after all the front end amplification and at the output of the first conversion there should be plenty of signal to see. Does that port need to be activated from a menu?

Carl

[w1vtp]

It should be sensitive enough to just hook up to the ant and see signals.  What's the IF freq you're looking for?  As pointed out by others the 50 ohm input may seriously load down many IF's.  What's the RX you are trying to use?

Yet another disadvantage is that if you are looking at a post filter part of the IF, you will be severely limited in BW displayed.  I'm betting that a SP-600 would deliver the IF to a 50 ohm input but I'll check tonight when I get home.  But again, even with the SP-600 there would be BW limitations.

The FET follower is another excellent suggestion - in fact, HP made a model 1120a FET probe that took its power off the front of the 8558 ( http://www.testelectronics.com/used/hp1120a.htm ).  There were TEE adaptors that you could stick the probe into to pick off a signal from a BNC connection.

GL  es 73 Al

[ke7trp]

I have hooked old and new SSB adapters to SP600s many times. They have a cap of the if tap already

[Opcom]

I've used older MF-5 frame Panoramic/Singer spectrum analyzers for this. They have a switch on the front of the VR-4 (50KHz to 27Mhz) plugin for 50 ohms or 1M Ohm. All the receivers tried have an IF jack of one kind or another.

[WQ9E]

For comparison, I believe my Yaesu YO-901 with pan adapter is representative of the sensitivity of typical "ham panadapters".  It specifies a sensitivity of 50 microvolts per 4 divisions in log mode or 500 microvolts per 4 divisions in linear mode.  My Kenwood SM-220 with BS-8 pan adapter has the odd specification of "more than 20db u/divison" which I cannot interpret.  The Yaesu has switchable 50 or 5K ohm input, Kenwood doesn't specify input impedance.  I didn't find any meaningful information for my Heathkit HO-13 and SB-620 displays.

In any case any usable test grade spectrum analyzer should easily monitor IF level signals and with higher gain settings you should see a lot of "grass" on the baseline from wideband IF noise.  The "baseline clipper" control will be useful in many cases.

[KC2TAU]

I tried this on three separate receivers, a Kenwood TS-850, a Kenwood TS-830 and an Icom R-70. All three have provisions for "scope" outputs on the back panel though these are more for connecting to their proprietary band scopes than to a spectrum analyzer. The TS-850 outputs the 8.83mhz IF, the 830 455khz and the R-70 outputs its 70mhz IF. I know the spectrum analyzer works well so perhaps the impedance mismatch is at fault. Perhaps that FET follower would help things.

[w1vtp]

I tried this on three separate receivers, a Kenwood TS-850, a Kenwood TS-830 and an Icom R-70. All three have provisions for "scope" outputs on the back panel <snip>

Those outputs were likely for Hi Z input scopes.  A FET HI Z to 50 OHM would fix that problem.  If the connectors on these receivers are RCA or BNC types - there is no magic - er propiertary stuff involved.  Just use a Z adaptor as previously discussed and center the 8558 on the IF center freq.. There will be BW limitations imposed by the RX IF filters.  Now, if you could hook things up pre-IF filtering, you might see more of a given band.

Al