Some practical design approaches

The literature on DCRs has several different popular approaches, anil each has its own place. Some of the simpler designs are based on the combination of a Signetics NE-602 device and a LM-386 IC audio section. Others are based on different

10 Rick Campbell, KK7B, -LHigh Performance Direct-C-onversion Receivers," QST. August i992 up 19-28.

11 ost August 1980, pp. 14-19; PaulG DauJton, K5WMS, "The Explorer; HF Receiver for 40 and Si) Meters." 73 Amateur Radio 7bday, August 1992, pp 30-rt4: John Dillon, WA^RNC, The Neophyie Receiver," QST, February 1988, pp. 14-18.

12 Radw-Et^rtrmiics, April 1990, pp 49-62; Joseph J, Carr, NE-602 Primer,' ElekWr Electronics USA, Jan. 1992, pp. 20-2 5.

IC devices, such as the Signetics TDA7000 or some other product. These chips were designed for the celluiar telephone and "cordless" telephone markets as receiver front ends. Still others are based on commercial or homebrew double-balanced modulators. This section examines several of these approaches.

The NE-602 type of DCR is relatively easy to build and provides reasonable performance for only a little effort.. The NE-602 chip is relatively easy to obtain; for the most part, it is well-behaved in circuits (i.e., it does what it is supposed to do). It has about 20 dB of conversion gain, so it can help overcome some circuit losses, and it reduces slightly the amount of gain required of the audio amplifier that follows. The NE-602 can provide very good sensitivity; on the order of 0.3 m-V is relatively easy to obtain, but it lacks something in dynamic range. Although the specifications of the device allow it to accept signals up to —15 dBm, at least one source recommended a maximum signal level of -25 dBm.14 At higher input signal levels, the NE-602 tends to fall apart.J 5 The newer NE-612 is basically the same chip but has improved dynamic range.

Figure 6-9 shows the basic circuit of the simplest form of NE-602 DCR. The input and LO circuits can be any of several configurations, although the one shown here is probably the most common.16 The output signal is taken from either pin 4 or pin 5 and is fed to an audio amplifier. This circuit configuration wiil work, but it is not recommended, There will be a fairly large noise and image signal component and no filtering.

TTie Dillon design shown in Fig. 6-10 uses the push-pull outputs of the NE-602 (i.e., both pins 4 and 5) and is superior to the single-endi^ variety. According to Dillon, the balanced output approach improves the performance, especially in regard to AM BCB breakthrough rejection. Also helping the breakthrough problem in the use of a 0.047-jiF capacitor across the output terminals of the NE-602,17

Dault.on lakes exception to the use of the NE-602 as the DCR front end and prefers instead to use the Signetics TDA-7000 chip. Although functionally similar to the NE-602, the TDA-7000 is more complex and is said to deliver superior performance with respect to dynamic range and signal-overload characteristics. Figure 6-11 shows a DCR front-end circuit, that is based on the TDA-7000 after Daulton's design. This circuit uses the same balanced front end as other designs and. like the typical NE-602 design, uses the internal oscillator for the variable-frequency oscillator (VFO). The circuit following this front end should be of the sort typically found in the NE-602 designs. This particular variant uses the internal operational amplifiers of the TDA-7000 to provide active bandpass filtering.

A simplified variant on the Lawellen design'8 is shown in Fig. 6-12 (the original design included a QRP transmitter as well as the DCR). The front end of this variant consists of an RF transformer with a tuned secondary winding (¿i.v), This secondary is tuned to resonance by capacitor and is tapped at the 60-ft point in order to match the input impedance of the double-balanced mixer (DBM).

14 Ibid. (Covington).

17 Telephone conversation between the author and John Dillon, 27 August ] 99i>. Sw aisy Irion's article fop. cti.).

IS Ibid. (Lawellen).

Gambar Product Detctor Ssb With Ne612

6-9 Partial schematic of NE-GQ2 direct conversion receiver.

The particular DBM selected here is a Mini-Circuits SBL-1-1, although in the original article, Lawellen used a homebrew DBM made from diodes and toroidal transformers. The RF signal is input to pin 1 of the DBM (which can accommodate signal levels up to +1 dBm), and the local oscillator signal ("VPO IN") is applied to pin 8 through capacitor C% The VFO/LO signal must be on the order of +7 dBm,

The design of Fig. 6-12 uses two methods for matching the output of the mixer circuit. First, there is an RC network (flj/63) that matches high frequencies to 51 ft (the capacitor limits operation to high frequencies). The second method used here is to use a grounded-base input amplifier (Qi) to the audio chain. Such an amplifier applies input signal across the emitter-base path and takes output signal from the collector-base path (the base being grounded for audio ac signals through capacitor C&). This preamplifier is equipped with an active decoupler circuit, con-sis( ing of transistor Qn and its associated circuitry. The input side of the grounded-base audio amplifier consists of a LC low-pass fiiter (CyRFCi) that passes audio frequencies, but not the residual VFO and RF signals from the DBM. In the original design, Lawellen followed the grounded-base amplifier with a direct-coupled operational amplifier active low-pass filter.

Antenna

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6*10 Complete schematic i>f direct-coi ivf-rsioit rnrejwr i^iiig NE-^02.

Antenna

Audio output

Antenna

Audio output

Double Balanced Antenna

6-11 Direct-conversion receiver using TDA-700Ü chip.

The Campbell design^ extends the concepts from La wellen. Figure fi-13 shows the block diagram for a portion of Campbell hs direct convex ion receive]'. The front end consists of a double-balanced mixer followed by a matched diplexei filter that provides a 50-il input impedance from dc to 300 Hz and from 3U00 Hz to some upper high-frequency Imyond the range of interest. The diplexer also passes t-h« standard communis m.ions ;uidio bandwidth (300 to 3000 Hz) lo Hie matched grounded base amplifier. Finally, there is an LC audio bandpass filter prior lo sending the signal on to the high-gain audio amplifier stages.

Figure 6-14A shows the passive diplexer used by Campbell. It consists of several ijidfictor, resistor, and capacitor elements that form both low-pass and high-pass filter sections. The values of the inductors {L\, and L.i) are selected with their dc resistance in mind, so it is important to use the originally specified components or

Ibfct (Campbell).

Tda7000 Ssb
input

6-12 Direct-conversion mixer and first audio using the SBL-1-1 DBM.

their exact equivalents in replicating the project. Campbell used Toko Type 10RB inductors; Ll is Toko 181LY-392J, L% is Toko 181LY-27&?, and Ls is Toko 181LY-273J. These coils are available from Digi-Key (P,0, Box 677, Thief River Falls, MN 56701-0677, USA; Voice 1-800-344-4539; FAX 218-681-3380).

Campbell's article supplied me with another example of the "digital myth," i.e., the concept that the digital implementation of a function is always superior to the analog version. He points out that the dynamic range of an LC filter is set by the inductors. The low-end is the thermal noise currents created by the circuit resistance (4KTBR), and the upper end is set by the saturation current of the inductor cores. For the parts selected by Campbell, he claims this range to be 180 dB. By contrast, an expensive 24-bit audio A/D converter provides only 144 dB of dynamic range.

The matched 50-fl audio preamplifier is shown in Fig. 6-14B and is an improved version of the Lawelien circuit. According to Campbell, this circuit provides about 40 dB of gain and offers a noise figure of about 5 dB. The range of input signals that it will accommodate ranges from about 10 nV to 10 mV without undue distortion. These specifications make the amplifier a good match to the DBM. Like the Lawelien circuit, the Campbell circuit uses a grounded-base input amplifier (Q|) and an active decoupler But Campbell also adds an emitter-follower/buffer amplifier (Qs).

A set of three passive audio filters, which can be switched into or out of the circuit, is shown in Fig, 6-14C. These filters are designed for termination in an impedance of 500 O. Three different bandpasses are offered: 1, 3, and 4 kHz. The 4-kHz

162 frirect^ommvion tndio receivers Antenna

6-13 Block diagram for a complex direct-conversion receiver.

filter is a fifth-order Butterworth design, while the 3-kHz filter is a seventh-order Elliptical design after Niewiadornski,20 The 1000-Hz design is scaled from the 3000-Hz design. Campbell claims that these filters offered a shape factor of 2.1:1, with a ex essentially flat passband ., with rounded corners, no ripple and no ringing."

Campbell implied the use of switching, as shown in Fig, 6-14C, but did not actually show the circuitry. As shown here, the switching involves use of a pair of ganged SP3P rotary switches, PIN diode switches can be used for this purpose.

A complex DCR was designed by Breed and reported in the amateur radio literature as a direct-conversion single-sideband receiver.The single-sideband (SSB) mode is properly called »ingle-sideband suppressed-carrier amplitude mi}d ula-iicn, for it is a variant of AM that reduces the RF carrier and one of the two AM sidebands to negligible levels. This mode is used in HF t ransmissions because it. reduces the bandwidth required by half and removes the charier that produces heterodyne squeals on the shortwave bands.

20 Rain Radio, Sept. 1985, pp. 17^30; cited in Campbell (up cit. )

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6-14 (A) Diplexer circuit for DBM; (B) audio postamplifipr: and (C> three audio bandpass filters.

Diplexer Design Diy

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