Figure 17 shows a schematic diagram of a generator that can be used to tune the intermediate frequency path in radio receivers for a wide variety of purposes. The frequency of the generator output signal - f f = 465 kHz * - is set by the quartz resonator ZQ1, and its amplitude - at least 2 V - depends on the power supply voltage Upit.
All resistors in the generator are MLT-0.125 type, KM-6 capacitors or the like. Transistor VT1 is almost any n-p-n, having a current gain of at least 100 and a cutoff frequency of at least 100 MHz.
Rice. 17. Generator for tuning the IF path of the radio receiver
The generator does not require adjustment. To maintain a good signal shape at 10 V, you may only need a few
increase the capacitance of capacitor C2 (up to 6200....6800 pF).
With such an amplitude of the output signal, the generator does not need to be connected to the radio receiver - it is enough to just bring them closer. But the output signal level can be reduced to bring it to the desired level. So, for example, as shown in Fig. 18. But in this case, the generator itself will need to be placed in the screen (a fragment of it is shown by the dashed line), otherwise interference “through the air” will not allow receiving a sufficiently low level signal at its output. With good shielding of all circuits, the resistor divider can be made stepwise (Fig. 19), the output signal of which can be reduced, if necessary, to fractions of a microvolt. The calculation of such divisors is described in.
Rice. 18. Simple output voltage divider
Rice. 19. Step output voltage divider
*) IF path carrier ff=465 kHz - domestic standard. In foreign communications technology, ff=455 kHz is more common. To configure such equipment in the generator, you only need to change the quartz resonator.
Universal generator-probe
Compact test signal generators are very popular among radio amateurs; they are useful for testing and setting up radio receiving and sound reproducing equipment. We offer another design of a similar generator, characterized by an expanded set of fixed frequencies.
Industrial and homemade radio receiving equipment contains 3F and IF paths, and the IF frequencies have different values: 455 kHz in imported and 465 kHz in domestic AM signal receivers; 5.5, 6.5 and 10.7 MHz - in FM signal receivers. The magazine "Radio" has already published circuits of generator-probes for testing 3Ch and IF paths. As a rule, they produce two signals - 3F and a modulated IF signal with one of the named frequencies. To avoid having to make several probes, the proposed generator provides frequency switching. It is suitable for testing almost any equipment, including the audio path of televisions.
The generator-probe circuit is shown in Fig. 1.
The audio frequency generator is assembled on transistor VT1 according to a circuit with a phase-shifting RC circuit (capacitors C1 - C4 and resistors R1 - R3). The emitter follower on transistor VT2 decouples the generator from the load - the RF generator. The latter is made on transistor VT3. Instead of resonant LC circuits, the generator uses small-sized piezoceramic IF filters ZQ1 - ZQ5 from radios or TVs. The filter corresponding to the desired IF is selected by switches SA1 (FM or AM) and SA2 (specific IF value). In the 3H position, no filter is turned on and the RF generator does not work. In this case, only the 3H signal is output.
The modulated RF signal is supplied to the output emitter follower, assembled on a VT4 transistor, which significantly weakens the influence of the load (the units being tested) on the RF and 3F generators. Variable resistor R8 sets the required output signal level. Separating capacitors C7 and C8 at the generator output are switched by button SB1. In the position of switch SB1 shown in the diagram, only modulated RF signals pass through capacitor C7 of a relatively small capacitance. When switches SA1 and SA2 are set to position “34”, use button SB1 to connect high-capacity capacitor C8. Power is supplied to the probe from the power circuits of the equipment being tested. The supply voltage can range from 3 to 12 V.
The generator-probe is assembled on a board made of getinax or fiberglass. The location of parts and connecting conductors are shown in Fig. 2. If the board is made of foil material, then a printed circuit board can be made according to the drawing. After manufacturing, the board is placed in any suitable housing, for example, from the GSP-1 grid field generator.
(click to enlarge)
Transistors VT1 - VT4 can be replaced with KT3102 or KT312 with any letter index; it is advisable to select transistors VT2 and VT3 with the highest current transfer coefficient. Any piezoceramic filters from domestic or imported equipment with suitable frequencies are suitable for the HF generator.
Switch SA1 is used type PD9-1, SA2 - PD21-2, button SB1 - MP-7 or other small-sized. All resistors are MLT-0.125 (you can use MLT-0.25), capacitors are KD, KM, K10 or other small ones. Resistor R8 - SPO-0.15 or SP-3-386. The output contact X1 is a needle soldered to a pad on the board (on the right in Fig. 2), and contact X2 is a wire with an alligator clip soldered at the end.
Setting up the probe generator begins with setting the mode of transistor VT1. Its collector voltage should be 1.5 V with a supply voltage of 3 V. To set the collector voltage, resistor R4 is selected. After this, the presence of generation is checked when the supply voltage changes from 3 to 12 V. Then capacitor C3 is unsoldered (the 3Ch generator stops working), a supply voltage of 3 V is applied and by selecting resistor R7, RF generation occurs at all fixed frequencies, i.e. when connecting any piezoceramic filter. If generation does not occur in any of the positions of switches SA1 and SA2 (most often this happens in position “10.7”), select resistor R6 and then again check the operation of the RF generator at all frequencies.
You can verify the presence of RF generation by connecting a high-frequency oscilloscope, millivoltmeter, a simple detector with a measuring head, or a frequency meter to the probe output. In the latter case, the generation frequency is also checked. Then install capacitor C3 in place and, if you have an oscilloscope, check the quality of the RF signal modulation.
Working with the probe is simple. If a 3H amplifier is being tested, switches SA1 and SA2 are set to the “3H” position, press the SB1 button and apply the 3H signal with probe X1 alternately to the various stages of the amplifier being tested, not forgetting to set the required signal level with resistor R8. When checking the amplifier of various equipment, select the required frequency value using switches SA1 and SA2; do not press the SB1 button. By applying a signal to the input of the amplifier, first after the main selection filter, and then before it, one is convinced that the signal passes through the filter and the amplifier. Otherwise, the UPC is checked step by step.
Literature
- Malinovsky D. Frequency synthesizer for the 144 MHz range. - Radio, 1990, No. 5, p. 25.
- Titov A. Probe-generator for testing radio receivers. - Radio, 1990, No. 10, p. 82.83.
- Nechaev I. Probe-generator for testing radio equipment. - Radio, 2000, No. 8, p. 57.
Setting up a radio receiver or the receiving part of a radio station is a rather complex process that requires increased attention and careful execution. The entire process of setting up a VHF receiver should be divided into three stages.
First, you need to check the correct installation and functionality of each stage, starting with the lowest frequency, i.e. you need to start from the “end” of the diagram.
Rough tuning of all oscillatory circuits included in the receiver. This setting should also start from the “end”. Tuning is usually carried out using a sufficiently strong RF signal of the required frequency applied to the receiver input.
Fine tuning of all receiver circuits, especially UHF. The tuning is carried out by applying a very weak, at the noise level, RF signal of the required frequency to the receiver input. The final step in tuning should be to measure and calculate the noise figure of the UHF receiver.
All these setup steps can be performed using homemade measuring instruments.
To carry out rough tuning of a VHF receiver or converter, you should apply a signal from a simple noise generator to its input. The diagram of such a simple device is shown in Fig. 1. You can also make and use a slightly more complex device, the diagram of which is shown in Figure 2.
Fig. 1 Schematic diagram of a simple noise generator:
Fig.2 More complex noise generator:
When setting the converter to 29 MHz or 145 MHz, immediately after connecting the noise generator to the UHF input, a noise signal will appear at the output of the receiver. Trimmers (capacitors) should achieve the maximum possible amplification of the noise signal.
Only rough adjustments can be made this way. Often this setting is sufficient. Fine-tuning the VHF receiver or converter and checking the directional properties of the antenna can be done using more sophisticated instruments.
Receiver fine tuning
As a result of fine-tuning the receiver, the maximum possible sensitivity of this receiving device should be achieved.
The sensitivity of the receiving device is one of the most important parameters that determine the potential capabilities of the entire work of the device creator. Therefore, objective methods for determining and comparing the sensitivity of various receivers, available for use in amateur (home) conditions, are of great interest.
The most accessible, and therefore the most common, way to determine the quality of a receiver is to listen to signals on the air. Obviously, the accuracy of such estimates is extremely low, since the signal level of a remote radio station can change tens or even hundreds of times.
Gennady A. Tyapichev - R3XB (ex RA3XB)
A simple amateur radio measuring device.
Homemade radio constructor.
Previously, an amateur radio tester included an intermediate frequency generator for tuning the receiver, but over time this function was no longer available for testers, however, the circuit of a simple measuring device may well be useful for tuning IF bandpass filters and troubleshooting in VHF (FM) receivers. A special feature of the circuit is that instead of quartz, a synthesizer and a processor, a 10.7 MHz piezoceramic filter is used, with the help of which not only frequency stability is ensured, but also its deviation is easily carried out by a tone signal, providing an FM oscillation at the output.
First I made the most simple FM generator-probe, where a low-frequency tone signal generator with a frequency of about 1 kHz is made on transistor T1, and a high-frequency generator is assembled on transistor T2. The frequency stability of the RF generator is ensured by a piezoceramic bandpass filter with a frequency of 10.7 MHz. The same filters are used in the intermediate frequency path of a radio receiver, and therefore, while conveniently assembling the receiver, you can also make a simple FM generator to test it. With the help of a varicap, when turned on in this way, under the influence of a 1 kHz tone signal, a frequency deviation of the order of +/- 25 kHz is ensured. The circuit uses a varicap (BB640), which has a large capacity.
A single-transistor tone generator, in addition to its simplicity, is capricious. The pure sinusoid at its output will greatly depend on the set mode, and, consequently, on the power supply and at the level of minimal nonlinear distortion, its operation will be unstable.
| Rice. 1 Simple FM generator-probe. |
Distortion of the tonal low-frequency signal can be avoided by ensuring reliable generation mode if you use an active low-pass filter (LPF) on an operational amplifier (op-amp). Thus, an additional 1 kHz tone output with attenuator is added to the RF probe to test low frequency amplifiers (LF amplifiers). Now the low-quality sinusoid, passing through the op-amp, will be cleared of higher harmonics, transforming into a pure low-frequency signal at the output.
When using different piezoceramic filters with a frequency band from 200 to 280 kHz, I concluded that more accurate tuning is obtained with filters with a narrow band.
The tuning frequency of the RF generator does not change when exposed to a modulating signal.
Parameters of the HF FM generator.
Supply voltage 3-5 V.
The generator frequency is 10.7 MHz, the error is 0 - 15 kHz.
Frequency deviation +/-25 kHz.
The output voltage into a 50 Ohm load is 500 mV.
Suppression of higher harmonics more than 30 dB
The RMS voltage of the low-frequency tone signal with a frequency of 1 kHz is in the range of 1.5 - 2 V.
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| Rice. 3. HF toy transmitter with FM. |
This RF generator, made according to the circuit in Fig. 2 is easy to change
into a simple FM transmitter- toy Fig. 3, since such a low-power output will provide wireless communication only within a few meters, which, however, is quite suitable for karaoke. The signal from the transmitter can be received by connecting an antenna to the IF input (10.7 MHz) of a receiver with a VHF band or to a receiver with a HF band. A generator based on transistor T1 and a low-pass filter on an op-amp, the DD 1 microcircuit is converted into an audio amplifier (ULF). An electret microphone-tablet is connected to the input.
The RF generator with AM is made on quartz 455 kHz (465 kHz).
The only difference is in the supply of the modulating signal Fig. 4. A 10 kOhm variable resistor changes the modulation depth, the maximum value of which in this circuit will be 30%. To obtain a deeper adjustment, it is necessary to change the value of the resistor R * to a value of 300 kOhm.
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| Rice. 4. RF generator with AM. |
We present to your attention a kit for assembling a simple 1 kHz + 465 kHz generator probe
The generator was developed by Sergei Eduardovich Belenetsky (US5MSQ). A detailed description of the design is posted on the author’s website here http://us5msq.com.ua In addition, there you can find information on its other designs, ask questions on the forum, and also purchase kits for assembly. This design is published with the kind permission of the author and, I hope, will be of interest to radio amateurs. Its schematic diagram is shown in the drawing below.
When repairing an audio amplifier or household radio at home, it often becomes necessary to trace the passage of the signal through the cascades. And this causes certain difficulties during repairs for radio amateurs who do not have the necessary equipment.
The simple generator-probe we bring to your attention is designed for repairing radio equipment. It does not contain winding units and can be manufactured, configured and operated even by a novice radio amateur. The probe generator allows you not only to check the serviceability of the audio amplifier and the intermediate frequency amplifier circuit (IF 465 kHz) of the radio receiver, but also to adjust the IF circuits of the radio receiver to the maximum signal level. The schematic diagram of the device is shown in the figure below:
A low-frequency generator is assembled on transistor VT1, producing oscillations with a frequency of approximately 1 kHz (determined by the parameters of the phase-shifting circuit C1C2C3R1R2 included in the OOS circuit).
The output signal is fed to the base of the VT2 RF generator through a single-stage low-pass filter R5C5, which cleans the output signal from harmonics and reduces its amplitude to obtain an AM modulation depth of approximately 30%.
The high-frequency generator operates at a frequency of 465 kHz and is made according to a three-point capacitive circuit (Clapp version), only instead of an inductor, a ZQ1 ceramic resonator is used. In this circuit, generation of oscillations is possible only with inductive reactance of the resonator circuit, i.e. the oscillation frequency is between the frequencies of series and parallel resonances. A small-sized ceramic filter FP1P1-61-02 (marking without color marks) was used as a resonator. FP1P1-61 filters of the FP1P1-61 series are widespread, not expensive and, most importantly, when switched on as indicated in the diagram, they have a small spread of parameters in the generation frequency, the actual spread in the generation frequency does not usually exceed ±0.5 kHz (according to the specifications it should not exceed ±1 kHz). Thus, when using virtually any filter from the FP1P1-61 series, it is guaranteed, without adjustment, to obtain a test signal with a frequency of 465 ± 1 kHz, which is what we actually need. The VT2 emitter is loaded onto a resistive divider R7R8, which reduces the output signal to practical levels and ensures stable operation of the generator regardless of the connected external circuits (device under test). Potentiometer R9 is used to smoothly adjust the output signal level.
With the right switch position indicated in the diagram, the output of the probe generator will be an AM signal with a frequency of 465 kHz, modulated by a low-frequency signal of 1 kHz (30% modulation). In the middle position of SA1, only a low-frequency signal with a frequency of 1 kHz will appear at the output.
You can use any RF transistors (KT315, KT3102, BC847, 2N2222, etc.) with H21e in the range of 100-220, otherwise you will need to select R4 to obtain 4.5 ± 0.5 V at the collector VT1.
The probe-generator is powered by a 9V battery from Krona.
The use of a small-sized three-position switch to switch operating modes made it possible to place the probe on a small board comparable to Krona-type batteries.The control assembly showed that in the absence of installation errors, the structure starts up immediately and does not require any adjustment, incl. and individual adjustment of the mode of transistor VT1 indicated in the diagram by selecting resistor R3 - because Since the sets use transistors from the same batch with a small spread in H12e (ranging from 300 to 330), the value of R3 remains unchanged.
Cost of a printed circuit board with mask and markings: 30 UAH
Set cost (p printed circuit board with mask and markings + full set of parts) to assemble the probe generator: 100 UAH