HF Signals

BITX40 - Circuit Description

- Ashhar Farhan, VU2ESE

The original BITX was published on the Internet in the year 2003. In the last 13 years, it has grown to become one of the most popular rigs among radio amateurs around the world. The BITX40 board is this very classic now available as a fully tested board that is easy to hookup, modify and operate. You can read the original article that described the BITX at http://phonestack.com/farhan/bitx20.html.

Download the circuit diagrams: Page 1 and Page 2

What's special about this version of the BITX?

Development Notes

Almost all modes of radio communications share a natural principle that the receivers and transmitters use the same line-up of circuit blocks except that the signal direction is reversed. The CW direct conversion transceiver is the simplest illustration of this principle. A more complex example is the bidirectional SSB transceiver.

Bi-directional SSB transceivers have been quite common in amateur literature. A transceiver was described in the ARRL SSB Handbook using bipolar transistors. W7UDM's design of bidirectional amplifier (as the basis of bidirectional transceiver) is referred to by Hayward and DeMaw in their book Solid State Design. The bidirectional circuitry is often complex and not approachable by the experimenter with modest capability.

The broad band bi-directional amplifier

My interest in bidirectional transceivers arose after looking at an RC coupled bidirectional amplifier in the book Experimental Methods in RF Design (p. 6.61). An easily analyzed circuit that was simple and robust was required. It began its life as an ordinary broad-band amplifier:

There are some interesting things about this circuit:

In order to make bidirectional amplifiers, we strap two such amplifiers together, back to back. By applying power to either of amplifiers, we can control the direction of amplification. This is the topology used in the signal chain of this transceiver. The diodes in the collectors prevent the switched-off transistor’s collector resistor (220 ohms) from loading the input of the other transistor. A close look will reveal that the AC feedback resistance consists of two 2.2K resistors in parallel, bringing the effective feedback resistance to 1.1K. All stages of amplification in this transceiver work this principle.

Diode mixers

The diode mixers are inherently broadband and bidirectional in nature. This is good and bad. It is good because the design is non-critical and putting 8 turns or 20 turns on the mixer transformer will not make much of a difference to the performance except at the edges of the entire spectrum of operation.

The badness is a little tougher to explain. Imagine that the output of a hypothetical mixer is being fed to the next stage that is not properly tuned to the output frequency. In such a case, the output of the mixer cannot be transferred to the next stage and it reflects back into the mixer. Ordinarily, if the mixer was a FET or a bipolar device, this reflected power just heats up the output coils. In case of diode ring mixers, you should remember that these devices are capable of taking input and outputs from any port (and these inputs and outputs can be from a large piece of HF spectrum), hence the mixer output at non-IF frequencies reflects back in the mixer and mixes up once more creating a terrible mess in terms of generating whistles, weird signals and distorting the original signal by stamping all over it.

A simple LC band pass filter that immediately follows the diode ring mixer will do a good job only at the frequencies it is tuned to. At other frequencies, it will offer reactive impedance that can cause the above mentioned problems. It is a requirement that the diode mixer’s input and output ports see the required 50 ohms termination at all the frequencies. In other words, they require proper broadband termination. Using broad-band amplifiers is a good and modest way of ensuring that. A diplexer and a hybrid coupling network is a better way, but it would be too complex for this design.

Circuit Description

Although simple, every effort was made to coax as much performance as was possible given the limitations of keeping the circuit simple and affordable.

The Receiver

The RF front-end uses a high performance 3 section band-pass filter for strong image and IF rejection. The three poles of filtering provide for a no-tune bandpass filter that needs no adjustment.

The 7 MHz bandpass filter. Each vertical division is 5 MHz

The RF Amps

An RF amplifier follows the RF band pass filter (Q1). There is 8mAs through the RF amplifier and the post-mix amplifiers to keep the signal handling capacity of the circuit above average. The Post-mix amplifier (Q2) does the job of keeping the crystal filter as well as the diode mixer properly terminated. The crispness of the receiver is more due to this stage than anything else. An improper post-mix amplifier easily degrades the crystal filter’s shape and introduces spurious signals and whistles from the diode mixer.

The VCO

The BITX40 uses a voltage controlled oscillator that covers 4.8 Mhz to 5 Mhz to cover the 7.0 Mhz to 7.2 MHz. A varactor is easier to tune as mounting a VFO capacitor properly is difficult and good quality tuning capacitors are no longer available. Those who like slower tuning rates could use a multiturn 10K linear pot instead of a regular potentiometer. The VCO is fed via a broad-band amplifier into the doubly balanced mixer. The trimmer provides exact band covereage. This oscillator has low noise though it does drift a little like all analog oscillators. It settles down to a very imperceptible drift within 10 minutes of warm up. Tip: Keep the BITX40 always turn on with the audio set to zero. This avoids the warm up drift. The receiver takes just 90 mA current.

The Crystal filter

The 12 Mhz crystal filter follows a Cohn topology. All the capacitors around it are 100pf. We use just 4 crystals to keep the ringing down and side-band suppression is 40 db. The receiver sounds exceptionally clean because of this crystal filter and the low noise VCO.

BFO, Detector and Audio

The BFO is a plain RC coupled crystal oscillator with an emitter follower. The emitter follower has been biased to 6V to prevent limiting. The detector also doubles up as the modulator during transmit mode; hence it is properly terminated with an attenuator pad. It has no impact on the overall noise figure as there is enough gain before the detector. The Q13 audio pre-amplifier is a single stage audio amplifier. The 100pf capacitor across the base and collector provides for low frequency response. The receiver does not have an AGC. This is not a major short-coming. Manual gain control allows you to control the noise floor of the receiver and I personally find it very useful when searching for weak signals or turning it down to enjoy the local ragchew.

Transmitter

The microphone amplifier is DC coupled to the microphone. This was done to steal some DC bias that is required when using an electret microphone that is supplied with the kit. The common Personal Computer type of headset too need this bias voltage. If your microphone does not require any bias, then insert a 1uF in series with the microphone. The microphone amplifier is a simple single stage audio amplifier. It does not have any band pass shaping components as the SSB filter ahead will take care of it all. One 0.001uf at the microphone input and another at the modulator output provide bypass for any stray RF pickup.

The two diode balanced modulator has no balance control. Most of the times, this control was actually causing carrier leakage due to bad contact and not being properly set. If you desire better carrier suppression, you can substitute the board's modulator diodes with another pair of 1N4148 with matched forward resistance. The attenuator pad at the output was found necessary to properly terminate the diode modulator and keep the carrier leakage around the IF amplifier to a minimum.

Rest of the transmission circuitry is exactly the same as the receiver. There is an extra stage of amplification (Q7) to boost the very low level 7 MHz SSB signal from output of the bandpass filter to 300mVs : enough to directly drive a driver stage.

Easy tune-up

The BITX40 is provided with everything pre-tuned. However, if you ever need to retune it, here is the procedure:
  1. The tune-up requires an ammeter (you can use your VOM set to high amps range) inserted in the power line to monitor the board's current consumption.
  2. Disconnect the PA power and the mic from the board
  3. Connect a well matched antenna with SWR of less than 1.5 to the antenna socket
  4. Switch on the rig to receive mode.
  5. On a frequency counter, measure the oscillator output at the collector of Q8. Rotate the tuning control to the low end of frequency and set the frequency to 5.000 MHz by tweaking the trimmer
  6. Tune in an SSB station and tweak the BFO trimmer for best sounding SSB.
  7. With volume of the rig set o minimum, the current consumption should be between 90 mA to 110 mA.
  8. Confirm that the PA and the mic has been disconnected from the board and flip the T/R switch to transmit mode
  9. The board's current consumption should rise to between 200 mA and 250 mA.
  10. Switch off the power, connect back only the PA power. Keep the (blue) bias preset (R108) completely clock-wise
  11. Switch to transmit mode, Now advance the bias preset in anti-clockwise direction until the board's current increases to between 300ma and 350ma.
  12. Switch off and connect the mic, Check the transmission

Getting Help

Join the mailing list for BITX on https://groups.yahoo.com/group/BITX20/ for all the help you need. You can ask all your questions, get suggestions and help with your BITX project within hours from the mailing list. See you on the band soon!