Log Detector Sweeper (II) Build

Oct/31 2016

This is the 3rd one of a series build blog:

Signal: Prototype of Log Detector Sweeper

Most important build notes deposited in to second one:

Signal: Improve The Log Detector Sweeper

Overall block Diagram

Achieved clean IF spectrum

the VCO connect to the DBM's LO port, in such a configuration, the VCO output level variation hardly affected the output level, which is linear than connect VCO to the RF signal port.

Another Big advantage is, RF now is been fixed, it's very easy to filter out from the output spectrum. DBM's RF rejection is surprisingly low. Most of time we would concert only the LO rejection, then i neglected RF reject of the DBM for a long time during debug period.

The IF port series with a RF trap which is turned to the RF frequency (61Mhz), this help the RF signal find a low impendance path to ground then eliminate the RF signal from IF spectrum.

Note, here we don't care the dynamic range, but want the "IF" signal to be spectrum clean. To achive this, the RF signal should be well eliminated ASAP. that's why the 66Mhz LPF is for. LO signal also should be spectrum clean. This is hard than the RF part, still, a simple 105Mhz LPF given a noticeable improvement.

Another factor to ensure 'IF' signal harmonics clean is that the RF level should be less than LO 15~20 dB, this is same as use the DBM in a receiver front end.

RF PAD is a key element to reduce non-harmonic distortion. IF there is RF leaking to IF even a little amount, the output will not only contain the 66Mhz in the output spectrum, there is also other lower frequency element generated by this leaking RF energy. For example, while IF signal sweep over 11Mhz, 22 Mhz, 33Mhz, IF signal's 6th, 3rd, 2nd harmonics will beating with this leaking RF energy, given a far more lower frequency spectrum element, it's very hard to eliminated after such frequency component been created.

VCO Strip

To minimize the harmonics, a 105Mhz LPF is connected to output of VCO, intended to reduce the harmonics of band 60Mhz-105Mhz. It's obviously, while VCO tuned to 64 Mhz the 2nd harmonics could not be well eliminated.

refer to: VCO: Improved Series E VCO , A 55-120Mhz VCO built to driven DBM mixer.

** tune the trim-capacitor to eliminate harmonics.

Reference build topology

2 cascaded buffer-amplifier stage

The buffer is a simple amplifier with very lightly coupled to VCO. And a same one use as amplifier connected after the buffer.

Insert Gain: 10 dB, bandwidth at least 100 Mhz.

BJT: 2SC9018, 800 Mhz Ft

Output transformer: 6T:2T on TV core

bias for VCC 8V ~ 11V: 8.2k 1.2k could be replaced by 6.8k, 1k combination.

degeneration resistor on the emitter is 47 ohm

5 order 105Mhz chebyshev LPF

The Air core inductor:

1 mm diameter enamelled wire. wound on 6mm Diameter , 4.5T spread to around 1 cm. stop frequency around 108.

Refer Air Coil 4.5 Turns example for how I wound the 4.5 turns coil.

Crystal oscillator Strip

Refer to:

CXO: Tune 5th Butler Overtone VHF Oscillator

64 Mhz 5rd Overtone Oscillator

Circuits changes:

2.8 Mhz crystal

Tank coil： 7T on pencil (240 nH) (better than white TDK coil former )

22 p change to 14 p, and 43 p change to 30 P+ 24 pf trimmer ( even 100p )

16.8pf tank capacitor replaced with a 24 pf trimmer

base decouple capacitor 104 change to 103

important note on the 15k POT:

For the POT resistance lower than 22K would cause parasitic oscillation at low frequency . Even quench the oscilloscope, the crystal could not control the oscillation frequency in such a situation. Here I replace this resistor with a 33 k.