a lull in the action

On Monday, I attended the monthly meeting of my local radio club where I picked up a portable oscilloscope from Steve W3AHL to assist in testing the Si570 frequency synthesizer’s output voltage. The past few evenings have been busy with a couple college basketball games of interest to watch (Kentucky on Tuesday, Duke last night) so I haven’t had a chance to work on the PSK-20 project.

I may warm up the soldering iron this evening and fire up the oscilloscope which W3AHL has graciously allowed me to borrow in order to check the voltage from the frequency synthesizer. According to the specs, the SA612AN expects a 200-300 mV signal from the oscillator. The stock output is much higher than that. W3AHL had suggested that I use a Pi attenuator to reduce the voltage from the synthesizer. I followed his initial advice, but without an oscilloscope I wasn’t able to check the voltage. Before I had the chance to test it, I had received this message from him:

The only problem is the 10db attenuator is wrong.  Last night I realized we were measuring voltage — I calculated the attenuation needed based on power ratio, which is what I normally use for RF circuits.   So just to make sure I got it right this time I verified what was needed using the 8924C’s signal generator and a 30 dB attenuator I had.

So to take the 2.2 volt P-P signal (unterminated — it would be 1.1 volts if the PSK-20 board had a 50 ohm terminator resistor on its end of the coax) down to the desired 250mv. signal, requires an 18.9 dB attenuator.  This would require two 62.8 ohm resistors to ground and a 217 ohm resistor in series between them.   Or as close as you can get in standard values.

The current 10 dB attenuator should give 700 millivolts output — a tad too much.

Again, I’m glad to have the experience and expertise of folks like Steve W3AHL to help with projects like this one. I’ll post results when I have a chance to test them–hopefully this evening.

Yesterday I ordered a low pass filter kit from Dieter “Diz” W8DIZ, who runs an awesome site/store called KitsAndParts.com. I explained to him what I was trying to do, and he offered to help me create a low pass filter with a cut-off frequency just above the output of the frequency synthesizer (around 5.3 MHz) to help transform the square wave from the synthesizer to a sine wave (which is preferred by the SA612AN).

John,
I made you a custom LPF for 5.3 MHz
Need to calculate the turns on the T50-2 cores
Caps are 470p and 1000p
All packed and ready to ship tomorrow AM
-Diz

Diz has always been helpful and mindful of his customers’ needs. This can be clearly seen in his reviews on eHam. Hopefully the parts will be in by the weekend and the next phase of the project can be tackled before the beginning of the work week.