The AM Forum

I have been running tests with Flex software and my homebrew SDR I/Q QSD converter. Stand alone with a crystal oscillator as a source I found it possible to monitor the phase noise from my 2 HP 8640Bs real time over more than 100 dB range. Then I connected the board to a Racal RA6830 front end and second mixer.  Now I could see the phase noise spectrum of the Racal synthesizer real time over a broad spectrum comparing it to the performance of the HP8640B.
the Racal synthesizer has a phase noise null pot that I have never been able to set because I didn't have anything to monitor the spectrum real time.
So the flex software and a converter like a Softrock makes a powerful tool and is quite accurate. Connected to the IF of a good RX makes it even more powerful.
Tie it to a good RX and you end up with better wideband dynamic range than a Flex radio. The Flex has better close in numbers at the expense of BB spurs though.
So even though the Flex software isn't so great pulling signals out of noise it sure makes a great spectrum analyzer and spectrum display. Not bad for the $15  softrock converter and free software. gfz

Frank,

In what region did the pot affect the phase noise?

the pot adjusts analog compensation
looks best after the 811 glow

Tom, The non wise a$$ answer is I don't know because my old HP141 scan rate is so slow at 10 HZ bw I can't see it. also the dynamic range is not wide enough. Now maybe I can see it. The 6830 synthesizer phase noise falls off quickly after about 2 KHz dBc and a good module is around -80 to -90 dBc at 1 KHz if the power supply is quiet. When you look at an HP8640B this shelf is about 15 dB better as you would expect with a cavity and divider chain. The crystal oscillator LO in my QSD converter is very quiet since it has a divide by 8 after it with local voltage regulator. This makes it a very useful tool interfaced with the HPSDR module. This can be scaled to any frequency with a good converter and mixer. The analog compensation is a DAC 08 AC coupled to the loop filter. It makes a series of high frequency pulses

Ah good test equipment. I was easily able to null the 6830 synthesizer phase noise using the flex software as a spectrum display. Now I have a tool to allow me to hot rod the synthesizer.

New front end constructed today with my best Racal hardware. Ran it on 75 tonight with a nice spectrum display of modulation both RX and TX.