DBM In The Frequency generator

Sept 21 2016

PLL or DDS synthesize Signal generator dominate the market (Fix me), unlucky they all looks like huge, and request digital knowledge as well as analog. i don't favor that for now.

i want a fancy sweeper for a very long time, and recent half year, i struggle to get one. cause un-familiar with the double balance mixer, i made so many fault, misunderstanding, frequently feel like get into a dead way.

basic idea is mix the VCO(60-105 Mhz) with a crystal oscillator (60 Mhz), get a 300 khz - 45 Mhz sweeper generator.

detail information refer to :

Signal: Prototype of Log Detector Sweeper

Signal: Improve The Log Detector Sweeper

Why RF signal Bad

1. LO drive level insufficient, and trying get more power from the DBM

2. RF level too high compared to LO

low band RF, good waveform

lower band RF signal, with pad or serials resistor, and change crystal oscillator

3. Top band of IF frequency is too close to the RF signal

rebuild everything, AM come back, select the proper RF level and RF harmonics.

4. RF harmonics generating non harmonic spur

move filter position, what the non harmonics spur look like

5. overlooked weakness of DBM: the RF-IF isolation

many many trying:

improve oscillator, insert good filter, change bias, change DBM, change configuration, drive power level

check the DBM use crystal oscillator, running simulation, I got the key factor which could explain almost everything... like these phenomenon:

1. AM like signal output

2. low frequency harmonic distortion

3. RF leaking VS non harmonics spur

4. design a hard to construct LPF

Key factor to get harmonic/non-harmonic free signal

1. RF level should be less than LO 15~20 dB ( reduce harmonics, reduce IMD, high RF control the DBM instead LO)

2. RF signal harmonics clean, 5th filter (filter out the harmonics, reduce the non harmonics spur )

3. Output filter should eliminate the RF leak very well ( DBM RF isolation is weak than -30 dB, leaking RF generating many non harmonics spur )

7 order chebyshev + wave trap, RF located above 1.5 time the corner frequency ( 45 Mhz stop filter alter by the wave trap to about 38 Mhz, 60 Mhz RF)

5 order elliptic+ 7 order chebyshev , 45 Mhz corner frequency, 60 Mhz RF, 1.3 times

4. wave trap for RF does help a lot to kill non harmonics spur, even in a high order filter

5. LO's harmonic reduce is also help to kill non harmonics spur, even with a 3 order filter.