I’ve been accumulating all sorts of parts in my junk box over a span of nearly a half a century. Some of this stuff was removed from war surplus electronics obtained from ham flea markets, and mail order surplus houses. Included are components from ARC-5 command sets, and other famous military radios, things like variable capacitors, IF transformers, vacuum tubes and sockets, transmitting mica capacitors, etc.
There are also commercial surplus inventories and discarded engineering samples from places I’ve worked at, flea market finds, and more recently bargains on eBay.
Vacuum tube oriented components can sometimes be repurposed for use in solid state designs, though they are usually too large to fit in as the current trend in construction is miniaturization.
Many of the solid state parts I’ve collected over the years, some of the transistors and integrated circuits that were once popular with the amateur home brew crowd have become obsolete. Still, having an ample supply of them, I would probably use them in a project or two. Some of these are still available today, but in SMT package versions of the now obsolete through hole variants.
Consider the design of a QRP HF transceiver. I’ve been gathering ideas from the designs of W1FB, W7ZOI, N6QW, DK7IH, EI9GQ, G4LFM, VK3HN, and others. I’ll go into the actual circuitry I’ve decided to use in later posts, for now let’s look at the available parts.
A transceiver is a device that functions as both a communications receiver, and a transmitter (obviously that’s where the name comes from). The assumption is that most of the circuitry can be used for both functions, ie: the transmitter is sorta kinda the receiver run backwards. (It’s not really that simple though).
The very first transceiver was built for what was then called the ‘ultra high frequency’ bands, back when hams were first trying to make use of the five meter band. A super-regenerative detector and one or two stages of audio comprised the receiver. On transmit, the detector circuit functioned as a modulated oscillator, and the audio stages functioned as a microphone pre-amp and modulator. A multi gang switch, or a bunch of relays handled the task of switching the necessary circuits around from receive to transmit modes. In the late 1960’s (it was in the 1967 version of the ARRL handbook), a version of this idea was presented for the 450 MHZ band, a 6CW4 nuvistor was used in the detector and transmitter, while two transistors were in the audio stages. Fully transistorized versions of the same concept were used in cheap, made in Japan, CB walkie talkies.
We’ll take a look at the design of the receiver portion of a transceiver first.
A Modern HF communications receiver is still based on the superhetrodyne circuit. We won’t consider SDR or software defined radios at this point, although even they do make use of the superhetrodyne principle, though the functions are implemented in software rather than hardware. Early receivers used a single conversion to an IF frequency that was usually lower than the frequency of the lowest band being covered. Multiple stages of radio frequency amplification along with several tuned circuits in a preselector attempted to reject image response. The gain distribution between the RF and IF amplifier circuits wasn’t optimal here, resulting in strong signals overloading the first mixer. Going to a higher frequency first IF would allow for not needing as many (or any) stages of RF amplification, but double (or even triple) conversion was needed to provide the necessary selectivity. It wasn’t until good piezoelectric (crystal) filters with steep slopes to provide selectivity at HF IF frequencies were available that receivers could provide the required selectivity before taking gain.
Some receivers still did make use of double or triple conversion. If a first IF in the VHF range is used (up conversion), a single low pass filter in the front end will allow for general coverage over the entire MF and HF range. IF frequencies between 45 and 75 mhz are common here. A moderate bandwidth roofing filter is used at this IF, usually around 5-6 khz wide, this allows for reception of AM and NBFM signals, as well as SSB and CW. A second conversion, down to a lower IF between 5-10 mhz follows, usually without much gain between conversions. Several IF filters are now provided at this second IF, usually a choice of 200-500hz for CW, 2100 – 2700 hz for SSB, and 3.5-6 khz for AM or NBFM. Finally, sometimes a third conversion is done down to a low frequency IF of 455 khz. The third conversion oscillator and the BFO oscillator are made variable. This allows for moving the signal between an overlap of the bandwidth of two roofing filters, one in the HF IF, and the other in the LF IF. By doing so, we can continuously vary the effective receiver bandwidth over a wide range. Until the advent of IF DSP, this feature was standard in many amateur transceivers.
Today the last IF of 455 khz is replaced by an even lower one between 15-50 khz. The processing at this frequency is handled in the digital domain by a Digital Signal Processor. The functions of the roofing filter, IF amplifier, product detector, noise blanker, AGC, beat frequency detector, and notch filter are all performed in software. If the rig doesn’t have to provide for general frequency coverage the up conversion to a first VHF IF frequency isn’t provided and the first IF is in the 5-10 mhz range. If general conversion is provided, then up conversion is available, but is usually only performed outside of amateur band receiver coverage, and sometimes for the transmitter chain. The Kenwood TS-590 uses this dual conversion chain process.
For our amateur designed and built transceiver will first consider only the receiver circuit chain, and then decide which elements will be either reused, or duplicated for the transmitter chain.
First of all, we need the input pre-selector. Once upon a time, this was a band switched set of coils and a multi-gang capacitor. The operator would have to peak these circuits (with a single knob) when changing bands, or making a large change in frequency within a band. Thanks to computer aided design we can now design band pass filters that can cover an entire band with reasonably steep walls at the band edges. No more manual re-peaking required! The coils can be slug tuned inductors in miniature cans, or they can be wound on toroid cores. In the latter case, we will use ceramic trimmer caps to get the filters dialed in just right, or we could use precision fixed capacitors (1% tolerance or better) and fine adjust the turn spacing on the toroids to dial things in. Two popular circuit topologies for such filters are top coupled parallel tuned circuits with link input and outputs, and series tuned sections with shunt capacitors between each section. The latter may have better circuit Q, but the former are easier to align with a grid dip meter. You can buy band pass filter kits from QRP Labs, and raw toroid cores can be obtained cheaply in bulk from kitsandparts.com.
An RF preamp isn’t necessary on the lower of our ham bands (160-30 meters) as atmospheric noise will overpower the internal noise of our first mixer. On twenty meters and higher a preamplifier may prove desirable as it will provide an improvement in the front end noise figure. In any case, being able to switch this preamp in and out as band conditions demand should be provided. The preamp should be able to handle large input signals, so the use of an RF power type transistor in deep class A operation (large resting current) is common. Parallel JFET configurations are also popular. 2N5109’s and J310’s are common here. The J310’s are becoming hard to find and expensive in through hole TO-92 packages, but are only $0.20 cents each in SMT packages when bought in 100’s. Another good choice for a preamp are the SMT ERA series amplifiers by Mini Circuit Labs.
Finally, dual gate MOSFETs make good RF preamps. Gain can be controlled via the upper gate, with signal on the lower one. Old time parts such as the RCA 40673 are becoming hard to find, but low voltage SMT variants are still available at this time.
There are lots of choices for the mixer circuits. Diode ring mixers, either the packages ones from MCL, or home made using toroid wound transformers and discrete diodes will give good performance. FET switches make good mixers. These parts are made in DIL SMT packages, and builders either use home wound toroids or MCL transformers. Single, double balanced, or H-mode configurations are used (with a 1, 2 or 3 trifilar wound transformers).
IC mixers are also used. The NE/SA 602/612 8 pin circuits combine a Gilbert Cell mixer and an oscillator circuit in a single package. The oscillator circuit supports VFO, VXO, or crystal oscillator configurations, or an external oscillator can be used. These devices can’t handle strong signals and having some form of input gain control ahead of them is a good idea. They do work well as the receiver product detector, transmitting mixer or balanced modulator stages where the signal levels can be controlled.
Another IC device that was once popular was the CA3028. This early device consisted of three transistors and resistors in a differential circuit configuration. The input signal was provided in a balanced mode between the two bases of the pair, and the output was taken between the two collectors. The oscillator signal was fed into the base of the lower transistor. While this device is no longer available (except from Chinese parts jobbers on eBay), you can make your own from discrete transistors and resistors.
Finally, there is the MC1496. This is also a Gilbert Cell mixer, but without the built in oscillator. It makes use of external bias resistors so the internal gain and resting current can be configured. It’s a stronger mixer than the NE602, and it makes a good balanced modulator, product detector, or mixer. It’s still being made, mostly in an SMT package.
The last circuit block I’ll describe now is the IF amplifier. The CA3028 was commonly used here in both cascode and differential circuits. Two AGC methods were used. Reverse AGC (increasing AGC voltage decreases circuit gain), and Forward AGC (increasing AGC voltage increases circuit gain). Forward AGC was applied to the lower transistor to control the current though the pair in both cascode and differential circuits. Reverse AGC was applied to one of the upper transistors which when in conduction mode would steal current from the other, bypassing the signal to ground.
A circuit that made better use of the Reverse AGC control in a differential circuit was the Motorola MC1350. The MC1349 was a higher gain version. These were very popular in ham receiver designs (it is used in the Elecraft K2). Motorola also made the MC1550 and MC1590 which were similar. All of these are discontinued today, though the MC1350 can still be found from surplus outlets.
Discrete IF amplifier circuits using cascode FETS, or a single FET and a BJT are popular. Forward AGC is applied to the upper transistor. With the above mentioned IC amplifiers becoming unavailable, the discrete transistor route is the recommended one.
More in a later post …….