07 September 2007

Comb Generator - measured

I built the circuit in about one hour. With an accurate selection of NOR gates usage, the signal can be routed through the 74HC02 simply bending pins inwards:

This build is a preliminary version to verify my assumptions (the theory). I used a 14.000 MHz XTAL and I didn't care to keep leads short or add bypass capacitors: the more noise the better.

The 100 MHz frequency counter could measure the 14 MHz signal at the oscillator but went crazy when picking up the output, showing 3.8 MHz or so. Perhaps the glitches were too fast?

On the spectrum analyzer the output looked like this:

45-150 MHz, marker 70 MHz

150-450 MHz, marker 210 MHz

Comb teeth are 14 MHz apart (XTAL frequency, not XTAL/2 as stated in a previous post). Measured harmonic levels are as follows:

MHz dBm
42 -14
56 -14
70 -13,4
84 -14,4
98 -12
112 -15
126 -13,3
140 -17
154 -16,5
168 -18
182 -20
196 -21
210 -23,3
224 -28
238 -28
252 -34
266 -35
280 -43

Note: 42 MHz is estimated.

Up to 150 MHz levels are quite constant. The further quick decrease is probably amplified by the attenuation of RG58 cable used for measurement.

I cannot explain the notch at 294 MHz (lambda ~= 1 meter), maybe some series resonance in the measurement setup? Or an expected zero on the spike spectrum? Higher harmonics are between -35 and 40 dBm up to 450 MHz.

Conclusion. Although the comb is obviously not leveled it is useful for comparative measurements on VHF/UHF filters:
  1. take note of levels without filter
  2. insert the filter between the comb generator and the spectrum analyzer
  3. tune the filter and measure harmonic levels and compare with step 1
Add-on. A small trimcap on the output of the delay gate may vary the output duty-cycle, thus modify the theoretical spectrum roll-off in case measurements fall in the ~300 MHz range.