Scope readings for the Crystal Filter & Bilateral Amplifier Board

Crystal Filter and Bilateral Amplifier Board

This document will outline the Lets Build Something Crystal Filter Bilateral Amplifier board. Included are several screen shots of the circuit taken into account both Tx and Rx modes.

One of the concerns I have is the effect the circuit has on the input signal.  I’ve included spectrum analysis where appropriate for consideration.  As well I have concerns with the Rx mode and will need to look into this further.

Input signals were adjusted to 1Vpp as per the reading on the scope. The signal generator amplitude needed to be adjusted in order to accommodate the different effect that the 2 input connections were having on the generator.  Frequency used was 4.915mhz.

Let’s start off with the Rx mode. Left to right when looking at the board. As you can see the signal is severely distorted. I checked the signal generator into a 50 ohm load to confirm that the output is in fact a sine wave. The circuit is having a dramatic effect. 

Below is spectrum analysis of the input waveform.  Fig. 12 is the waveform.

Schematic and board layout is at the end of the posting.

Fig.11

Fig.12

Fig. 14, A is the toroid out and B in input to the toroid. Just under 200 mV in and 600mv out.

Fig. 15, A is the filter in and B is the filter out. Probes are directly on the in and out of the crystal. Bypassing the 39pf capacitors in other words.  Signal cleans up nicely.

Fig. 16 has A on the input of the transistor and B the output of the board.

 So the signal cleans up again and puts out 600mV 

Questions I have:

1, Why does the circuit distort the signals as it does in a couple of places?

2, Why the poor amplification?

It will be interesting to see what others have going on with their boards.

Ok, onto the Tx mode.

Fig. 2, the input. Note the flattening of the sine wave at the bottom. The circuit behaves much better here than the Rx circuit.

Fig. 1, Spectrum display. 

Fig.4, B is the input to the transistor and A is output. That is one messed up output.

Fig. 5, Spectrum display. Note the 2^nd harmonic.

Fig.6 bored yet?  Another shot of the transistor output

Fig. 7, B is toroid in and A is out.

Fig.8 Xtal in is A and B is xtal out.

Fig.9 is A toroid in and B is toroid out. Signal look ok and relative values as well.

Fig. 10, A transistor in and B is transistor out.

Tx mode, while it has its moments earlier in the circuit, seems to work ok once the signal is through the crystal filter.

The left side of the board is a little different in layout, but I don’t think it would be an explanation of the differences between the Rx and Tx mode signal behaviours.  Below is a copy of the layout. The board shows an IC in the centre, this is where the relay goes.