Accessories and Test Gear

Five Digit Frequency Counter

A counter is probably the most useful addition to any rig! The five digit version has two input channels which are normally connected to the rig's VFO (60+ MHz) and CIO; these can be read individually, added or subtracted so making it suitable for additive or subtractive superhets with any IF, or for direct conversion rigs or test gear. The five displays show the MHz & KHz with a normal LSB of 1 KHz. This counter is ideal for multi-band superhets. It uses high efficiency small 0.3" wide by 0.5" high common cathode 7 segment LED displays; these are driven by DC signals from the electrically quiet CMOS counting logic using a crystal reference oscillator whose harmonics are outside any amateur band. The individual LED segment resistors are mounted immediately behind their display and are separate from the driving logic PCB. The high impedance input circuits have a typical sensitivity of 50 mV RMS at 20 MHz. PCB size 160 x 50 mm. The kit includes parts for one input buffer and suits all Somerset Range rigs. Five digits £45.

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The Three Digit KHz Counter

This is a low cost alternative that normally shows just the important KiloHertz numbers of the input frequency - any MHz numbers overflow (normally) out of sight to the left! This means that it is suitable for direct conversion rigs, and for superhets whose IF is an integer number of MHz - as in my kits. The least significant digit shows the units of KHz, and the top digit shows the hundreds of KHz. It uses the same devices as the 5D counter so has all the advantages of low noise and DC currents in the wires to the displays. For use with superhets, it can count either up or down, with remote DC control for multi-band options. Input sensitivity is about 50 mV p-p at 10 MHz falling to 100 mV at 35 MHz; the maximum input frequency will be normally be over 60 MHz. The PCB size is 50 x 80 mm; supply required, 7 to 22 volts. The 21 segment resistors are mounted behind the display or end-on at the logic PCB. Two sorts of display are available - the narrow 0.3" wide type or the larger 1" high versions - please specify when ordering. The photo below shows the prototype fitted to the new Digital Sig Gen - together these kits make an excellent combination.  Extra MHz displays can be either hard wired, or made to count (under the control of the 3D logic PCB) by adding only two ICs. (This kit is not suitable for the Mendip.) The basic 3D counter cost is £30. Extra MHz displays & resistors £1 each, or two displays with 2 counting chips £5.

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Digital Signal Generator

This kit has two types of output; either a square wave or 'marker pips'. Both depend on digital techniques using 5v high speed CMOS logic. The square wave outputs are derived from the main digital VFO which runs between 6.5 and 16 MHz, with six switch selected ranges of frequency division, covering 130 KHz to 16 MHz. Main tuning is by PolyVaricon with a potentiometer Fine control. These square wave outputs are rich in odd harmonics which can be used right into VHF! Thus the third harmonic (the strongest) of the top range would cover 19.5 - 48 MHz, and the fifth 32.5 - 80 MHz etc.. Although it is unusual for a sig gen to have square wave outputs, for most applications they will be fine - for driving a mixer, the harmonics are unlikely to be a problem (and can occasionally be useful!) and for aligning filters, the harmonics will be removed by the filter anyway! The important thing is to be aware the harmonics are there and to consider if any funny results might be due to them (unlikely!). The alternative 'marker pip' outputs are derived from a 5 MHz crystal oscillator, followed by the same switch selectable dividers giving 'pip' spacings of 5, 2.5, 1.25 MHz, 500, 250 or 100 KHz. These are intended for calibrating the receiver tuning scales - the choice of 'pip' interval makes it much easier to be certain about the frequency of any individual 'pip'! The square wave output level is selected by another switch with nominal levels of 200, 20 and 1 mV p-p open circuit from a 50R source impedance; the pip output levels are much lower as the very narrow pulse energy is spread over a very wide spectrum up into VHF! The whole unit runs off a PCB mounted PP3 battery. If the Dig Sig Gen is combined with the 3D counter it becomes a very versatile piece of test equipment, especially if the counter can be switched between sig gen or an external signal. The Dig Sig Gen PCB has provision for two extra counting stages so that it becomes a 5 digit counter as in the photo below. (This is an early model without the extra switches!) The Dig Sig Gen is £36, or with the 3/5D counter and extra toggle £70.

Antenna Matching Unit

This unit includes a resistive 50R matching bridge, a broadband RF transformer and the T configuration matching parts. Power is limited to around 20W by the variable capacitors! When selected for tuning up the matching controls, the bridge will always present a safe load to the TX no matter what the antenna looks like! The bridge uses a high intensity LED for indication but can also work with conventional meters or an S meter, or the Audio Extras kit. When the bridge is out of circuit the LED indicates output RF voltage. The broadband RF transformer allows the matching section to work with balanced or unbalanced feed lines. The T matching section (two variable capacitors and a tapped inductor) is very adaptable, dealing with load impedances from 10 to 2000 Ohms. To avoid winding the main inductance, it is made from a printed circuit flexible strap with switch selected taps for 10m upwards - for 160m there are toggle switched extra capacity and inductance. Price £36.

This is the AMU, with the matching bridge resistors on the extreme left:-

Linear Amplifier

This is a broadband design for any of the 1.5 Watt transmitters of Somerset Range rigs - especially the Cam, Brue, Brendon, Bridgwater and Parrett. Producing 10W on all bands, 20 to 160m on 13.8 volts, it can also be used on the higher bands with reduced output. With higher supplies to 22 volts, it can produce appreciably more output power! By connecting it just after the driving rig's output transistor(s), it makes use of the rig's existing TR switching and low pass filtering, which reduces the total complexity. The bias supply is also controlled from the main rig. The PCB is only 80 x 100 and has two 50 mm high upright heatsinks. The photo below shows it attached to the rear of a Parrett, with the Tone underneath the Mini CW kit and Mini AGC alongside! Price £24.

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Mini CW kit

This unit adds CW facilities to an existing phone transceiver. For the receiver, it has a humped 725 Hz low pass audio filter, which can be selected from the front panel switch (included) to reduce the rig's normal phone audio bandwidth to that desirable for CW. The filter is normally connected just ahead of the rig's manual or automatic audio gain control stage. For the transmitter, it includes semi break-in TR control from the key input (with about a half second delay before it reverts to reception) and is connected to the rig's PTT circuit. Closing the key also activates the separate adjustable level 725 Hz tone outputs (from an on board sinusoidal 725 Hz oscillator) that are fed to the receiver for RX sidetone monitoring, and to the TX's speech amplifier stages to make it transmit a steady carrier while the key is down. It is suitable for DSB or SSB phone rig's. The PCB is single sided, 50 x 80 mm, needing a nominal 12 volt supply. In the photo immediately above, it is the small PCB on the front end with two presets in the middle. Ideal for the Tone/Parrett or the Bridgwater/Burnham combinations! The price is £18.

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Mini AGC kit

This unit provides audio derived Automatic Gain Control for receivers that have only a manual audio gain control. It is intended to be added immediately ahead of the rig's normal AFG control. It holds the audio output reasonably constant at about 250 mV p-p for inputs over about 15 mV p-p up to a few volts! It also has protection from the disturbing effects of noise spikes, which can lead unwanted desensitization of the receiver. There are no extra front panel controls. When fitted, the rig's normal muting (during transmission) needs to be altered to instead apply muting to the input of the AGC kit, so as to allow a quick recovery on reverting to reception. The PCB is single sided 50 x 80mm and needs a nominal 12 volt supply. In the photo immediately above, it is the PCB on the side of the Tone RX. The kit is particularly suitable for RX's with simple audio output stages that can be prone to overloading on very strong signals. Price is £14.

Mini Mixer kit

This design has two main uses; firstly for obtaining stable local oscillator signals on the higher bands, and secondly to act as a receiving converter. It has a SA602 mixer with crystal oscillator followed by a buffer stage, and a double tuned RF filter using toroids and trimmers. For crystal mixing LO applications, an external VFO feeds one mixer input with the other being provided by the mixer chip's internal oscillator using an appropriate crystal to achieve the desired higher frequency VFO range - by addition or subtraction. For receiving converter use, the aerial feeds the RF filter which then feeds the mixer and outputs to the main receiver. The standard kit includes parts for all bands 20 to 80 of the RF filter; the crystal is chosen to suit either the external VFO or the receiver input band - these would often both be 80m. The kit can easily be used with other crystals and RF filter frequencies. It occupies about three-quarters of the 50 x 80 mm PCB. Remember you need to define the application when ordering! Price is £14.

Speech Processor

This kit increases the average output level of an audio signal from a microphone by clipping off the excessive signal peaks. Because it is the peak signal which determines the maximum allowable drive to a phone transmitter, by lowering the peaks in proportion to the average level, a higher average output level can be attained with an associated increase in intelligibility under difficult conditions. The kit works by modulating a 5 MHz carrier which is then clipped, the resulting harmonics (of 5 MHz) are filtered out and then demodulated back to audio. It is set up easily without special equipment and has a preset for a compression control which can be changed to a normal pot if preferred. It can be used with any make of rig - requiring only signal connections to and from the normal mike socket. It uses a single sided 50 x 80 mm PCB working on nominal 12 volts. Price £24.

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Two Tone signal source

This kit comprises two sine-wave audio oscillators that can be used singly as a source of audio test signals or together as a pair of unrelated tones for accurate setting up of single sideband transmitters. The two fixed frequencies are nominally 725 Hz and 1590 Hz. A three position slide switch allows either oscillator run separately or their outputs to be combined. The outputs (approx 1 volt) are balanced with a preset using an ordinary AC multi-meter and then a further optional resistor is added to bring the level down to about 10 millivolts - a bit higher than most modern microphones, with actual output level being set by a further shafted preset. This low level signal is input to the transmitter under test while the RF output is monitored with an oscilloscope. The presets can easily be changed to normal potentiometers if required. Size is 50 x 80 mm on a single sided PCB. It can use a 9 volt battery (to avoid hum loops) or nominal 12 volt supply. Price £19.

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