by Roiand L. Guard, Jr. K4EFI

The DSB transmitter described herein uses crystal control and runs 1 watt or so, which can be used barefoot or to drive an rf amplifier.

The unit should be assembled in a small minibox or built on perf-board or PC board and then installed in a small minibox, as stray hand capacitances can upset the carrier balance.

The transmitter consists of speech amp, carrier oscillator, balanced modulator, and PA stages.

The amount of carrier suppression available with a diode-type balanced modulator is -40 dB.

Care should be taken in selecting the diode pair. Check the forward resistance of several diodes with your VOM until you find two with the same or nearly the same forward resistance. Germanium diodes were used in this unit which were in a grab-bag pack of 50 for $1 from Poly-Paks. The diodes should read at least 10:1 (forward-to-reverse resistance ratio).

Capacitors C2 and C3 are 30 pF variables. Cb is a variable, and capacitor CI is a 4—30 pF trimmer. Coils LI, L2, and L3 are 'A in. slug-tuned types removed from a TV PC boards many types of these coil forms

Fig. 1. This block diagram shows the simplicity of the homebrew JOm DBS transmitter. The tenth-watt unit can be used to drive a low-power linear or, for QRP fun, it can be used barefoot.
Dsb Transmitter
Fig. 2. Schematic diagram of DSB transmitter for 10m.

were tried and, although the ferrite slugs vary in Q, all coils resonated at 10 meters by juggling the slugs and tuning capacitors settings.

After checking the transmitter for operation by listening for the signal on a receiver, peak LI and CI for maximum S-meter strength. Then null out the carrier (minimum S-meter indication) with pot Rlr L2 and C2 may also be varied to null out the carrier. With the carrier nulled out, speak into the microphone with the mike gain control half open. You should be able to hear the double-sideband signal in your receiver. Next adjust L3 and C3 for maximum output.

At the home station, 1 use an RCA WO-88A oscilloscope to monitor the 50 kHz i-f strip in my Mohawk receiver. Observing the scope with the signal tuned in on 10 meters makes balancing the carrier a cinch. If you don't have a scope, monitoring your S-meter is sufficient.

On my scope, the carrier suppression of this transmitter is sufficient to put it way below noise level on the 10 meter band.

The carrier balancing adjustments should be carried out with the rig installed in its minibox, cover on. Small holes should be drilled in the appropriate places for adjusting the coils and trimmers. Changing crystals may upset the carrier balance and you will have to make the balance adjustments again. A few times and this becomes child's play.

Today's receivers can receive a DSB signal with no difficulty. Most of the time, operators will not be aware you are using DSB. This rig could easily drive a 6146B which would give you about 100W PEP.

This rig can also be used on any other band by changing the coils and crystal. To do this, "borrow" coil data from other published articles for the band you want. A homebrew vfo could also be built for this rig. This rig could be made into a walkie-talkie or hidden in the glove compartment of a car. It would be just the thing for talk-in at hamfests.

You can squeeze more power from Q2 by reducing the resistor values shown or by applying more voltage to Q2 collector (not to exceed the rated voltage for the particular transistor you use). However, you could also drive Q2 into a nonlinear operating condition. For DSB, as in SSB, you must operate the PA in its linear operating zone.

By turning the Rj, balance pot to either side, you can use low-level AM. K4EPI"


Douglas Stivison WB2MYU

YSu might say that The Radio Amateur's Handbook is crystal controlled — very little drift from the standard. Year after year the Handbook appears with basically the same format, pictures, tables, and charts. As a guide to the avant-garde in ham technique and construction, the Handbook is certainly lacking. Yet it remains the cheapest and most popular handbook of standard amateur construction and communication techniques.

I built my first CW rig from the Handbook, and my first VHF project, and my first homebrew test equipment, and I used its tables to put up my first antenna, and to figure out the color codes on all those weird surplus components. Dog-eared and smudged illegibly on the dozens of oft-used pages, with burn holes from hot solder globs, and with schematics traced in pencil, my 1963 Handbook has been consulted countless times. I've built no less than six rigs from it. I've never purchased a newer Handbook because the book just doesn't change much from year to year.

However, I think I might invest in the 1971 Handbook.

The Handbook, printed this year on off-white nonglare paper, is not the guide for the experimenter using the newest components and techniques. Rather, it is a compendium of standard circuits and immutable tables and charts. It is the ham's, if not the engineer's, reference for the time-tested circuits and the time-tested techniques. The majority of its projects have been fabricated time and again by hams. I have never built anything from the Handbook and had it work the first time — but every project has worked eventually. For the standard Q-multiplier, mixer, modulator, oscillator, or power supply circuit, the Handbook is the reference. Certainly it is not the only source for these

Was this article helpful?

0 0
Disaster Preparedness Kit

Disaster Preparedness Kit

Don't get caught out in a disaster, be prepared for the unlikely. You just never know when disaster will strike, but if it does you will be more then prepared for it with the information this ebook gives you.

Get My Free Ebook

Post a comment