This dual-purpose converter is designed to provide very good communications performance - plus a sensitive, stable and convenient amateur TV receive facility in conjunction with any UHF TV set.
The block diagram is set out in Fig 1. For communications operation, an output in the 2 m band allows the use of any popular 2 m rig as a tunable IF. The TV output - around channel 52 - permits the use of an unmodified UHF TV set. A single local oscillator and a broadband double balanced mixer provide IFs of 144 MHz and 720 MHZ. Table I gives details of the crystals which may be used, together with their various applications. For TV use, use the 97.33 or 98 MHz crystal to alleviate the major problems caused by the harmonic relationship of 144/288/ 432 MHz. A "junk box" crystal may well work for TV use, since the absolute frequency value is unimportant (TV sets do not, as a rule, come calibrated in MHz). The two crystals should be within 4 MHz of each other, although if slightly inferior oscillator spurii are acceptable this figure may be increased. If one of your local TV transmissions occurs on Ch 52, the second crystal can be selected to shift the IF frequency and avoid breakthrough - use of the optional input helical filter will also assist - although the filter losses will instantly compromise your noise figure by the degree of insertion loss (3-4 dB) - so a preamp is virtual must for serious DX work. The bandwidth required for TV must also be borne in mind, or definition may be lost if the RF bandwidth is too narrow.
Components List and Component Placement
Circuit Description
The input stage is a low noise UHF PNP transistor, the BFT95 (AEG, inter alia). One of the main advantages of a PNP device at UHF is the simple way in which the collector load is returned directly to ground (via the filter). Low inductance decoupling of the emitter is essential and by virtue of this capacitance from emitter to ground, a degree of low frequency roll off can be established. The optional input filter has already been mentioned. Where it is not used, the space on the board may be populated with a simple high pass filter to alleviate the unwanted attentions of 27 MHz. The filter tap points are at 50 ohm impedance, and thus suitable for direct connection to the mixer. Ideally, such mixers should be terminated with a resistive load to maintain best intercept performance, but this is not likely to compromise this unit, since the mixer is primarily employed for its wide band characteristics.
The local oscillator chain provides a choice of two crystals to cover the entire 70 cms band within the scope of a 2 m receiver's coverage. 5th overtone crystals are not generally the friendliest of quartz devices, and frequently tend to disappear on some obscure parasitic resonance unless carefully cajoled onto the right frequency. The resonant circuit established by L 1/C 1103 must therefore be reasonably reliably pre-settable, so TOKO S 18 molded coils are used to avoid ambiguity. L 2 is placed in parallel with the crystal to enforce overtone operation. Note that switching is performed at DC. Switching crystals is distinctly bad news and should be avoided. The system employed here enables remote operation if required. The multiplier chain uses a ZTX327 in the output, driving a bandpass coupled filter which produces a clean LO drive to the mixer. The mixer requires a high level (+ 7 dB) injection, and the ZTX327 or ZTX3866 are necessary to achieve the required gain and power. In view of the broadband nature of the mixer, it is important that the LO should be kept free from excessive spurii, or various unexpected mixing processes will occur. Careful decoupling is arranged throughout, and the whole unit is built into a screened box with capacitive feedthrough terminations.
- Perform the usual visual checks for solder bridges and incorrect insertions.
- Adjust the cores of L 1 and L 3 to about 2 mm below the top of the formers. The core of L 5 should be level with the top of the former - and C 15, C 16 to mid position (slot in line with the pins).
- Connect a 10 V power supply (preferably one with current limiting at 100 mA), and check that the current consumed is not excessive. It should be around 18 mA with neither crystal oscillator connected.
- Connect a test meter between Q 3 emitter and earth. An initial voltage of about 1 V should be observed. Earth the centre of C 35 to turn on 01
- Connect a test meter to Q 4 emitter. Adjust L 1 until a reading is obtained and adjust L 5 for maximum reading for Q 2 after earthing C 36 centre. Switch between Q 1 and Q 2 whilst adjusting L 5 to ensure an even level on both frequencies.
- Using either an RF millivoltmeter of diode probe, adjust C 15 and C 16 for maximum RP voltage at the output tap on L 8. With the oscillator chain correctly aligned, the current consumption will have risen to approx. 28 mA
- If additional test equipment is available (i.e. spectrum analysers), further adjustment of such things as F 1 and F 2 may be undertaken.
L7 & L8 Tap Detail
L5 top, L7 & L8 Below
