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AMATEUR RADIO By GARRY CRATT, VK2YBX Modem ICs make RF designs far simpler than earlier discrete transistor circuits This month, we are describing some simple home-brew converter designs based on the Signetics NE602 low-power VHF oscillator/mixer IC. Three circuits are presented: one for VHF and two for the HF bands. Last month's article covering the design and construction of a VHF frequency converter prompted many enquiries about a simpler design. And although we thought that the design presented wasn't unduly complex, we were tempted to find alternatives. Our target was to find a device which lent itself to home-brew design and which was both economical and readily available. Thanks to Signetics, there is a simpler way to frequency conversion - the NE602. This is an 8-pin DIL package IC containing a local oscillator and a double balanced mixer. Designed for VHF receiver front ends, the local oscillator will operate to 200MHz while the mixer is rated to 500MHz. Fig.1 shows the internal layout of Fig.1: block diagram of the NE602. It contains a local oscillator plus a double balanced mixer which is fed by a differential amplifier. the NE602. The combination of the differential amplifier and the mixer is known as a "Gilbert Cell". This arrangement provides up to 18dB of conversion gain at 45MHz, with a noise figure of 5dB at that frequency. At 45MHz, the IC is capable of rece1vmg input signals of -119dBm (0.3µ.V) for 12dB S/N, which means that it can easily be used on 6 metres. In addition to good VHF performance, the NE602 has very low power consumption, making it ideal for portable equipment. The IC has an onboard regulator and draws only 2.5mA at 6 volts. The input, output and oscillator ports can support a variety of configurations, the complexity generally being related to the performance. Fig.2 shows three different input circuit configurations. The input (pins 1 & 2) pins are internally biased and have .an input impedance of 1.5k0 in parallel with 3pF. The mixer outputs (pins 4 & 5) are also internally biased, allowing direct termination and also allowing for balanced output. Fig.3 shows four different output configurations three single and one balanced. Using a balanced input reduces harmonics, while a balanced output provides far superior suppression of the input RF and LO signals. Table 1 shows the advantages of ..... Y) ._____. a. Single-Ended Tuned Input b. Balanced Input (For Attenuation of Second-Order Products) c. Single-Ended Untuned Input Fig.2: three possible input configurations for the NE602 oscillator/mixer IC. 82 SILICON CHIP TABLE 1 Input Pins 1 & 2 Output Pins 4 & 5 ADVANTAGES DISADVANTAGES Single-ended No sacrifice in 3rd-order performance, simplified circuit Increase in 2ndorder products Balanced Reduced 2nd•order products Impedance match more difficult to achieve Single-ended Simple interface to filters 3dB reduction in output, less RF and LO isolation Balanced 3dB improvement in output, better LO and RF isolation at the output More complex circuitry required single ended and balanced terminations. Generally, the balanced configuration offers performance advantages but at the expense of circuit complexity. Local oscillator The NE602 on-board local oscillator is an emitter follower circuit and can be used in many different configurations. Pin 6 (oscillator base) and pin 7 (oscillator emitter) do not need any external bias circuitry. However, only pin 6 may be connected to DC supply. Pin 6 can also be used for an external oscillator or for frequency synthesiser injection. The on board oscillator can also be used 12pF liµH - ..rnlpf CFU455 OR EQUIVALENT NE602 FILTER K&L38780 OR EQUIVALENT •CT MATCHES 3.5KO TO NEXT STAGE. b. Single-Ended Crystal Filter a. Single-Ended Ceramic Filter NEI02 NEI02 c. Single-Ended IFT d. Balanced Output Fig.3: the output configuration can be either single ended or balanced, but the latter provides superior suppression of the input RF & LO signals. NE602 a. Colpitts Crystal Oscillator (Overtone Mode) NEI02 NE602 b. Colpitts L/C Tank Oscillator c. Hartley L/C Tank Oscillator Fig.4: three possible oscillator configurations. The Colpitts oscillator (a} operates in overtone mode, with L1 & C3 used to suppress oscillation at the fundamental frequency. OCT0BER1990 83 ,------------+9V ANTENNA 1k .01 ~ L2 L4 47k .,.v. .,.:B::f ;,:; IC1 NE602 OUTPUT TO RECEIVER Fig.5: this simple VHF converter uses a varicap diode to vary the local oscillator frequency so that it can tune the nominated band . The output can be tuned using a multimode communications receiver. +9V L1,3,5 : 2T, 25 B&S 3mm ID. AIR CDRED L6 = 0.2uH (approx). as a buffer for external injection. Fig.4 shows several oscillator configurations. Version (a) shows a crystal controlled Colpitts oscillator in overtone mode. Inductor 11 and C3 suppress oscillation at the fundamental frequency. Typical values for 50MHz or so are C1 = 5.6pF, C2 = . 22pF, L1 = 0.5-1.5µH and C3 = 1nF. The crystal is a third overtone type, and must be specified as a parallel mode, 5pF loading crystal. Best results are achieved when pin 6 is driven to 200-300mV peak to peak, The NE602 has some obvious advantages, including very low power consumption for very good performance. There are also some characteristics which are not so obvious. As a result of the very fast bipolar process used by the NE602, the phase integrity through all three ports is superb. This aspect makes the IC an ideal choice for image rejection mixer applications. And although Signetics specifies the device for use up to 200MHz, the NE602 has been used to 900MHz (although Signetics won't guarantee performance beyond 200MHz). Simple VHF converter Fig.5 shows a simple VHF receiver using the NE602. This particular design uses a varicap diode to allow the user to vary the LO frequency, so that the receiver can tune the nominated band. With the addition of an FM IF strip, the receiver would be quite useful on the VHF and UHF amateur bands. The addition of a simple AM diode detector would result in quite a good air band receiver. Alternatively, by feeding the output to a multimode communications receiver you would get the best of both worlds. Of course, the NE602 is not restricted to VHF or UHF use. The IC can also be used as a frequency converter at HF. Fig.6 shows a simple up-converter which will allow long-wave signals in the 300- 500kHz band to be received on a shortwave receiver tuning 4.34.5MHz. Fig.7 shows a direct conversion receiver for the 40-metre band. Both designs use easy-to-obtain IF transformers in the front end tuned circuit. However, any transformer covering a suitable frequency band could be used. Further reading (1). Signetics Linear Data and Applications Manual Vol.2, 1985 (2). Radio Electronics magazine, April 1990. (3). Signetics Application Note AN1981: New Low Power SSB Circuits. (4). Signetics Application Note AN1982 (5). Signetics Application Note AN198: Designing with the NE602. Footnote: the NE602 is available from Stewart Electronic Components, PO Box 281, Oakleigh, Vic. 3166. Phone (03) 543 3733. 300·500kHz 330pF Tl 455kHz IFT 47pF ;--!¥'--, ,1i;,-~1_ _ _____, 330pF I I VC1 I IC1 N£602 .001 ~OUTPUT 4.3MHz-4.5MHz .,. Fig.6: use this up-converter circuit if you want to receive long-wave signals in the 300-500kHz band. The output can be tuned using a shortwave receiver (4.3-4.5MHz). 84 SILICON CHIP IC1 NE602 .,. Fig.7: here's a design for a direct conversion receiver for the 40-metre band. The output can be fed directly to an audio amplifier.