RF Chain Details

This is a little bit closer shot of the 70MHz Low Pass Filter and the 10MHz crystal calibrator oscillator.

Notice on the 70MHz filter that the capacitors hold up the coils. I found that these coils are not all that critical. Initially, I was concerned that there might me some bleed through from high powered FM stations, and did actually notice some at FDIM (apparently there were several strong transmitters in the area). I haven't noticed any in use at my station at home, even though I'm in the near field (within 3 miles) of a very strong AM broadcast transmitter. It might have been better practice to put a baffle between the parallel coils, and it would actually have been better to not have any coils in parallel, but sometimes you just do what you have to do...

The 10MHz crystal calibrator oscillator is a very simple Colpitts oscillator, with a pi attenuator pad (to get the signal strength down to about -10dBm. The schematic is available on one of the notes pages. I actually built the circuit using the PCB layout for the NOGANaut transmitter, using a Dremel tool to cut the circuit traces. I added one extra copper pad so that I could used a 78L12 voltage regulator to insure stability and drop the +15V supply down to 12V (12V is indicated in the schematic). I put the pi pad where the pi filter goes on the NOGANaut.

The purpose of this oscillator, by the way, is to give you a nice signal every 10MHz (just like you used to do with a 100KHz calibrator osciallator. The unfiltered Colpitts oscillator is rich in harmonics, which show up very nicely in the SA. This circuit is, therefore, very valuable in identifying where in the spectrum (frequency-wise) the signal you are looking at is located.

Now, here's an interesting exercise. The pad is supposed to set the amplitude of the oscillator output to -10dBm. The output waveforem of the oscillator, with all of it's harmonics looks kind of like a row of teeth--approaching a square wave output, but not quite. So here's the question. If you set the amplitude of the waveform that you see in the screen to -10dBm, what is the amplitude of the fundamental frequency? In other words, can you use the crystal calibrator to determine relative amplitudes in the SA? The practical answer is, no. I had a very interesting conversation with Roy Lewellan, W7EL, at FDIM on the subject. We agreed that you could use the oscillator to calibrate frequency, but not amplitude. To calibrate amplitude, use a nice clean sine wave of a known frequency and amplitude and determine the relative amplitudes based on that calibration.