Wednesday, 23 August 2017

Two Essential Add-ons For Bitx and Other QRP Rigs.

In this post I am going to discuss two very simple but very essential add-ons for Bitx and other similar type of QRP sideband rigs. These two add-on modules are the recent addition to my XENA and BITX rigs that have already proved their worth during portable QRP operations. The first one is of course a simple speech processor. It can give more punch to your transmissions and more QSO's are certainly assured for you.

A SIMPLE SPEECH PROCESSOR:  A simple speech processor can enhance your readability on the receiving end by as much as two S-points and is an indispensable addition to any QRP voice rig. There are usually two types of speech processing techniques. One using limiting through compression and other through clipping. The latter is usually known to give better results on both AF and RF. The present circuit uses this technique. The signal from the single stage mic amplifier is routed through this module. The first stage gives it necessary amplification and the signal is clipped using a pair of back to back diodes. I used germanium diodes as these were available in my collection but BAT 54 or similar are likely to give better results. The signal is then filtered using an op- amp based sallen key filter for harmonic suppression and is then amplified and is routed to the diode DBM. The circuit can be use with almost any SSB/DSB rig. For use with bitx you should use 47K resistance for R1 or use a potentiometer at the output to get proper level of audio for modulation.  The circuit is very simple and is given below:

I have chosen an LM358 for being cheap, readily available and for its smaller footprint than discrete devices, to aid smaller design. You can play with the values of C1 and C4. Try a value between 0.0033uf to 0.0068uf for C1 to suit your voice quality. Generally the least bass input to the DBM is the key to best readability. Try 1nf for C4 for much less splatter across the transmitted bandwidth.

The alignment is simple. An oscilloscope can be indispensable but if you have none don't worry. Measure the average AF signal amplitude at DBM input using a sensitive AF meter. Now switch to compress mode and set the viper of R8 all the way to ground. Now adjust R4 for an equal and almost similar amplitude reading at the input of your DBM. Go on the air and call a friend. Adjust R8 now for best results......and you are all done.

A COMPACT ANTENNA TUNER: A simple and compact antenna tuner is made of just two components; C2 and L3. L3 can be wound on T-50-43 toroidal core. A 5 turn winding is good to cover both 80 and 40 meters. For 30 and 20 meters, three turns are sufficient and for twenty through fifteen meters just two turns suffice. C2 is a common BC type variable capacitor and is not hard to find. I usually use a half wave throw away wire for my portable QRP use as it is very convenient to carry.

This compact tuner can tune both high and low impedances very effectively and is small enough for portable use. I have included N7VE's SWR bridge for ease of tuning. The entire module is small enough to be built and accommodated within the bitx cabinet.


  1. 1. In the past I did some experiments with a clipping circuit using LTspice, using my own voice as the input to the circuit and listening to the output file of the LTspice. As I noticed, what is essential in the clipping circuit is to NOT clip the longer wavelengths (i.e., the lower frequencies). The clipping of the lower frequencies covers/hides/destroys the information existing in the higher frequencies. So BEFORE the clipping circuit, a sufficient low-cut filter should exist.
    2. Because of the increase of the RMS power, perhaps a reduction of the PEP power (by a couple of dBs) is useful/necessary.
    The improvement on the speech intelligibility with a suitable clipping circuit is amazing!

    Thank you for your blog!
    73 de SV1GAP

    1. Hi OM Christopher, yes I agree with your point that low frequency signal when clipped hard is just square waved and when it is filtered; phase relationship of the harmonics are altered causing signal peaks to be generated and getting added to the signal that can reduce the effectiveness and intelligibility. Consequently, as I mentioned the entry of bass to the audio chain should be limited. You can use smaller value of C7 for this. Also only a moderate level of compression that can give good crest level should be used. My experience is that around 10 to 12 dB of compression suffices for such AF processors.

  2. Dear OM. Alan, E means ohms. For example 10E=10 ohms.

  3. Two questions:
    1. R4 is shown as 47k ohm but I am unfamiliar with the symbol used in the schematic. It looks somewhat like a variable pot, is that what it is supposed to be?

    2. Have you seen anyone do an Eagle layout of these designs to make a board? Or some other software design tool that is compatible with Eagle?



    1. Dear K4GDL, R4 is a preset. It sets the gain of op-amp and thus the output level. I don't know someone who does Eagle job. I built my prototype on a piece of prototyping veroboard.