A7A3

A7A3 M/N Phase Detector, Circuit Description

The M/N phase detector has two programmable frequency dividers: an M divider and an N divider. M and N are integer numbers which give the ratio of divider input frequency to divider output frequency. This is the divide number. The input to the N divider is 20 MHz; the M divider input is the difference frequency between the M/N VCO (355 to 395 MHz) and 400 MHz. The two divider outputs are compared in a phase/frequency detector. The detector output is amplified and applied to A7A4 M/N Output where it is used to tune the M/N VCO.

In general, the M/N output frequency (from A7A4) is (200 to 10 M/N) MHz. The M/N VCO frequency is twice the M/N output frequency. The spectrum analyzer frequency diagnostics (displayed by (SHIFT) [MKR~REF LVL)) show the M number, N number, and M/N frequency.

The fifth line of diagnostics contains three integers; the second integer is the M number, and the third integer is the N number. The next (sixth) diagnostic line gives the M/N output frequency in MHz (177.5 MHz to 197.5 MHz).

TTL + ECL Level Translators @

The numbers to program the frequency dividers come from Al2 Front-Panel Interface in binary at TTL levels. U3, U13, and U17 shift these to ECL levels which are approximately -0.9 V logic high and -1.7 V logic low. Nl and Ml designate the least significant bits.

N Divider @ and M Divider @

The M and N dividers are virtually identical. Basically, the four most significant bits of N (three bits for M) N3, N4, N5, N6 load a counter U6. It counts down to two and is reloaded on the next clock pulse. The two least significant bits, Nl and N2, control pulse swallowing logic which causes the counter to count down to 1 instead of 2 before reloading. The logic determines the proper number of extra pulses to be swallowed to effect the proper divide number.

For troubleshooting, it is helpful to set the M and N numbers equal in a 1 MHz span using the frequency diagnostics explained above. Waveforms at corresponding points in the two dividers can then be compared to pinpoint the problem.

Mixer @)

Q3 is an amplifier which drives the LO port of the mixer Q24. It supplies about +5 dBm over the 355 to 395 MHz range. The output of the mixer is the difference between 400 MHz and the M/N VCO frequency which gives an IF frequency between 5 and 45 MHz. The IF level is about -17 dBm.

A7A3 1