Universal IF amplifier

The IF amplifier in Fig. 8-5 is based on the popular MC- 1350P integrated circuit. This chip is easily available through any of the major mail order parts houses and many small ones. It is basically a variation on the LM-1490 and LM-159G type of circuit but is a little easier to apply.

If you have difficulty locating MC-1350P devices, the exact same chip is available in the service replacement lines such as ECG and NTE. These parts lines are sold at

Vniixtrsal" IF amplifier 197

Antenna Inductance
+ 12 VDC

S-4 Cascode JFET IF amplifier.

S-4 Cascode JFET IF amplifier.

Amplifier Circuit

GAlk&DHlPOL

8-5 IF amplifier with crystal filter.

local electronics parts distributors and are intended for the service repair shop trade, I used actual MC-1350P chips in one version and NTE-746 (same as ECG-746) chips in the other without any difference in performance. The NTE and ECG chips are actually purchased from the sources of the original devices and then renumbered.

The circuit is shown in Fig. 8-5. TW MC-1350P devices in cascade are used. Each device has a differentia] input (pins 4 and 6). These pins are connected to the link windings on IF transformers (e.g., T2 at device £/, and at £/2). In both cases, one of the input pins are grounded for AC (i.e., RF and IF) signals through a bypass capacitor (C% and C4),

In the past IVe had difficulties applying the MC-1350P devices when two were used in cascade. The problem is that these are high-gain chips and any coupling at all will cause oscillation. I've built several really good MC-1350P oscillators—the problem is that I was building IF amplifiers. The problem is basically solved by two tactics that I'd ignored in the past. This time I reversed the connections to the input terminals on the two devices. Note that pin 4 is bypassed to ground on U^, while on lh it is pin 6. The ether tactic is to use different value resistors at pin 5.

Pin 5 on the MC-1350P device is the gain control pin. It is used to provide either manual gain control (MGC) or automatic gain control (AGC). The voltage applied to this pin should be between +3 and +9 V, with the highest gain being at +3 V and nearly zero gain at +9 V.

The outputs of the MC-1350P are connected to the primaries of 7a and T4. Each output circuit has a resistor Cf?2 and across the transformer winding. The transformers used are standard "transistor radio" TF transformers provided that the impedance matching requirements are met.

The DC power is applied to the MC-1350P devices through pins 1 and 2, which are connected together. Bypass capacitors C^ and are used to decouple the DC power lines and thereby prevent oscillation. All of the bypass capacitors (C?, C4, and Cs) should be mounted as close to the bodies of V\ and U > as possible. They can be disk ceramic devices, or some of the newer dielectric capacitors, provided of course that they are rated for operation at the frequency you select. Most capacitors will work to 10.7 MHz, but if you go to 50 MHz or so, some capacitor types might show too much reactance (disk ceramic devices work fine at those frequencies, however).

The dc power supply should be regulated at some voltage between +9 and +15 Vdc. More gain can be obtained at +15 Vdc, but 1 used +10 Vdc with good results. In each power line there is a 100-ft resistor (Hi and which help provide some isolation between the two devices. Feedback via the power line is one source of oscillation in high-frequency circuits.

The dc power line is decoupled by two capacitors. C- is a 01 -jjlF capacitor and is used to decouple high frequencies that either get in through the regulator or try to couple from chip to chip via the dc power line (which is why they are called "decoupling" capacitors). The ot her capacitor (Cs) is a 10- to 100-m-F device used to smooth out any variations in the dc power or to decouple low frequencies that the 0 1 |tF doesnt take out effectively.

The RF/IF input circuit deserves some comment-1 elected to use a double-tuned arrangement. This type of circuit is of a category that are coupled via a mutual reactance. Various versions of this type of circuit art1 known, but I elected to use the version that uses a capacitive reactance (C,) at the "hot" end of the IjC tank circuits. Coupling in and out of the network is provided by the transformer coupling links.

The power and gain control connections are bought through the aluminum box. wall through 1000-pF fe^dthrough capacitors. Two kinds are available, one solder-in

"Universal" IF amplifier 199

and the other screw thread-mounted. For aluminum boxes the screw thread is needed because it is difficult to solder to aluminum. Both types are available in either 1000- or 2000-pF values, either of which can be used in this application. If you elect to use some other form of coiuieetor, then add disk ceramic capacitors (0.001 y,F) to the connector, right across the pins as close as possible to the connector. There are several ways to make this circuit work at other frequencies, if you want to use a standard IF frequency up to 45 MHz or so, then select one with the configuration shown in Fig. OS (these are standard).

If you want to make the circuit operate in the HF band on a frequency other than 10.7 MHz, then its possible to use the 10.7-MHz transformer. If the desired frequency is less than 10.7 MHz, then add a small value fixed or trimmer capacitor in parallel with the tuned winding. It will add to the built-in capacitance, reducing the resonant frequency. I don't know how low you can go, but IVe had good results at the 40-m amateur band (7-7.3 MHz) using additional capacitance across a 10.7-MHz IF transformer.

On frequencies higher than 10.7 MHz you must take some more drastic action. Take one of the transformers and turn it over so that you can see the pins. In the middle of the bottom header, between the two rows of pins, there will be an indentation containing the tuning capacitor. It is a small tubular ceramic capacitor (you may need a magnifying glass to see it well tf your eyes are like mine). If it is color-coded you can obtain the value using your knowledge of the standard color code. Take a small screwdriver and crush the capacitor. Clean out all of the debris to prevent shorts at a later time. You now have an untuned transformer with an inductance right around 2 jiH. Using this information, you can calculate the required capacitance using this formula.

where

C is the capacitance in picofarads

/is the desired frequency in kilohertz (kHz)

L^h is the inductance in microheruys.

Equation (8-1) is based on the standard L-C resonance equation, solved for C, and with all constants and conversion factors rolled into the numerator. If you know the capacitance that must be used, and need to calculate the inductance, then swap the L and C terms in Eq. (8-1).

If the original capacitor was marked as to value or color-coded, then you can calculate the approximate capacitance needed by taking the ratio of the old frequency to the new frequency and then square it. The square of the frequency ratio is the capacitance ratio, so multiply the old capacitance by the square of the frequency ratio to find the new value. For example, suppose a 110-pF capacjfor is used for 10.7 MHz, and you want to make a 20.5-MHz coil. The ratio is (10.7/20,5)3 = 0,272 MHz, The new capacitance will be about 0.272 X 110 pF = 30 pF. For other frequencies, you might consider using homebrew toroid inductors.

A variation on the theme is to make the circuit wideband. This can be done for a wide portion of the HF spectrum by removing the capacitors from the transformers and not replacing them with some other capacitor, In that. ease. IF filtering is done at the input, (between the IF amplifier and the mixer circuit).

Crystal and mechanical filters require certain coupling methods. Figure 8-6 shows a method for coupling to a crystal filter connected between two bipolar transistors. Each stage of the amplifier is a common emitter bipolar ti<msistor amplifier, biased by R\/8£ and tfn./AV The connection to the filter circuit is direct because the filter is not sensitive to dc (such cannot be said of mechanical filters).

8-6 Universal IF amplifier.

Figure 8-7 shows a different approach that accommodates mechanical filters as well as crystal filters. The particular circuit shown is for very high frequency IF amplifiers (e.g., 50 MHz), but with changes to the values of the components, this IF plifier could be used from VLF through VHF regions. The resonant frequency of (.his circuit is set by and C, and the filter circuit. The amplifier is a MOSFET transistor device connected in ttu> common source configuration, (late G] is used for the signal and Gi is used for DC bias and gain control (see AGC").

The filter is connertcd to the filter through a capacitor to block the dc ;it tin-drain of the MOSFET device (similar capacitors would be used in hipolnr circuits us well). The output of the filter may or may not he capacitor couple«i depending upon the design of the circuits to follow this one.

Coupling to other filters

Mechanical Filter Circuit

Figure 8-7 shows a different approach that accommodates mechanical filters as well as crystal filters. The particular circuit shown is for very high frequency IF amplifiers (e.g., 50 MHz), but with changes to the values of the components, this IF plifier could be used from VLF through VHF regions. The resonant frequency of (.his circuit is set by and C, and the filter circuit. The amplifier is a MOSFET transistor device connected in ttu> common source configuration, (late G] is used for the signal and Gi is used for DC bias and gain control (see AGC").

The filter is connertcd to the filter through a capacitor to block the dc ;it tin-drain of the MOSFET device (similar capacitors would be used in hipolnr circuits us well). The output of the filter may or may not he capacitor couple«i depending upon the design of the circuits to follow this one.

to demodulator

IC IF amplifiers

The universal IF amplifier presented earlier is an example of an integrated circuit (10) IF amplifier, hi this section we will take a look at several additional IC IF amplifier circuits.

Amplifier Circuit Repair

8-7 MOSFET IF amplifier With AG€

MC-1590 circuit

Figure 8-8 shows an amplifier based on the 1490 or 15&0 chips. This particular circuit works well in the VHF region (30 to 80 MHz), Input signal is coupled to the IC through capacitor C|- Tuning is accomplished by Cs and Lu which forms a parallel

INPUT o

INPUT o

Cascade Amplifier

OUTPUT

OUTPUT

8-8 MC-1590 IF amplifier.

resonant tank circuit. Capacitor C:i sets the unused differential input of the 1590 chip to ground potential, while retaining its dc level

Output tuning in Fig. 8-8 reflects the fact that the 1590 chip is differential output as well as differential input. The LC-tuned circuit, consisting of the primary of and capacitor C&, is parallel-resonant and is connected between pins 5 and 6. A resistor across the tank circuit reduces its loaded Q, which has the effect of broadening the response of the circuit.

V+ power is applied to the chip both through the V+ terminal and pin 6 through the coil ¿2. Pin 2 is used as an AGC gain-control terminal.

SL560C circuits

The SL560C is basically a gain block that can be used at RF and tF frequencies, figure 8-9 shows a circuit based on the SL560C. The input of the SL560C is differential, but this is a single-ended circuit. That requires the unused input to be bypassed to ground through capacitor C3, Because this is a wideband circuit, there is no tuning associated with the input or output circuitry. The input circuitry consists of a 0.02-|fcF coupling capacitor and an RF choke (RFCi).

FROM MIXER

FROM MIXER

OUTPUT

8-9 SL-S60C IF amplifier

A tuned circuit version of the circuit is shown in Fig. 8-10. This circuit replaces the input circuity with a tuned circuit (TO and places a transformer (TJ in the output circuit. Also different is that the V+ circuit in this case uses a zener diode to regulate the dc voltage.

Dozimetr Pin Diodou

6-10 SL560C IF amplifier (tuned circuit version).

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