Progressive Receiver Design -
Direct Conversion Receiver / Product Detector
Notes:
- The value of the 1uF capacitor in series with the AF output from the CA3140 will be determined by the input impedance of the following stage. The CA3140 op-
amp will not drive very low impedance phones. - Gain of the op-
amp as shown is approximately x10 (20dB). Gain of mixer/detector is around x6 (16dB). More AF gain may be required for a Direct Conversion receiver and less for a product detector. Try it and see. - Roll-
off of the RC network between the mixer and op- amp should start around 3KHz. Shunt filter capacitors should be polyester, decoupling capacitors can be ceramic. - A preselector is not shown, try a one section tuned circuit to start. Note that input impedance of NE602 is around 1K5 so do not connect to the top of a tuned circuit but tap down.
- C1 will be around 47pF for a crystal oscillator. Feedback capacitors between A, B and 0v will be around 150pF for 9MHz. The values for other frequencies and the tunable version will depend on the frequency and band coverage that is required.
- Do not let the voltage on pin 8 of the NE602 exceed 8v. The 820ohm resistor should provide sufficient voltage drop but a 6v low power regulator could be used instead and would be more predictable.
- No Automatic Gain Control has been provided so some form of RF attenuator may be required.
- This detector is only suitable for SSB/CW.
- When used as a product detector with preceding amplifier stages the gain of the CA3140 op-
amp may need to be reduced or the CA3140 could be left out entirely and the audio output from the NE602 connected directly to the audio filter mentioned in the next paragraph. - Some low frequency and high frequency roll-
off in the audio range is provided by this circuit but additional frequency shaping is recommended for best results - see the additional AF Band Pass Filter which is part of this project. - The 9v supply must be hum free. See the power supply sheet for suggestions.
- All resistors should be 5%, 250 or 330mW carbon film type.
- 1n and 10n should be 10%, 50vdc ceramic single or multi-
layer types. Note that +80- 20% capacitors are suitable for de- coupling but not for audio filtering or coupling. 10n may be polyester. - Pins 1, 5 and 8 on the CA3140 should be left unconnected.
- The CA3140 is a CMOS device that is susceptible to damage by electrostatic charge, handle with care.
- Keep the wiring around the oscillator section (NE602 pins 6, 7) screened or well away from the aerial wiring (pin 1) to reduce the amount of oscillator leakage reaching the aerial and irritating other local band users.
- This circuit should work with medium to high impedance (600-
2000 ohms) headphones but will require an additional amplifier for use with low impedance (4 ohm) headphones or a loudspeaker. - The NE602 may be replaced by the SA602.
