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A crystal filter is, as you know, "half a good transceiver". This article presents the practical design of a twelve-crystal quartz filter of the main selection for a high-quality transceiver and set-top box for a computer, allowing you to configure this and any other narrow-band filters.

Recently, in amateur designs, quartz eight-crystal ladder-type filters made on the same resonators are used as the main selection filter. These filters are relatively easy to manufacture and do not require large material costs. Computer programs have been written for their calculation and simulation. The characteristics of the filters fully meet the requirements for high-quality signal reception and transmission. However, with all the advantages, these filters also have a significant drawback - some asymmetry in the frequency response (flat low-frequency slope) and, accordingly, a low squareness factor.

The workload of the amateur radio determines rather stringent requirements for the selectivity of a modern transceiver in the adjacent channel, so the main selection filter must provide attenuation outside the passband of at least 100 dB with a squareness factor of 1,5-1,8 (at -6/-90 dB levels). Naturally, the losses and uneven frequency response in the passband of the filter should be minimal.

Guided by the recommendations set out in [1], a ten-crystal ladder filter with a Chebyshev characteristic was chosen as the basis with a frequency response unevenness of 0,28 dB. To increase the steepness of the slopes, additional circuits were introduced parallel to the input and output of the filter, consisting of series-connected quartz resonators and capacitors. The parameters of the resonators and the filter were calculated according to the method described in [2]. For a filter bandwidth of 2,65 kHz, the initial values ​​were obtained С1,2 = 82,2 pF, Lkv = 0,0185 H, RH = 224 Ohm. The filter circuit and the calculated values ​​​​of the capacitor ratings are shown in fig. 1.

(click to enlarge)

The design uses quartz resonators for television PAL decoders at a frequency of 8,867 MHz, manufactured by VNIISIMS (Alexandrov, Vladimir Region). The stable repeatability of crystal parameters, their small dimensions and low cost played their role in the choice. The selection of the frequency of quartz resonators for ZQ2-ZQ11 was carried out with an accuracy of ±50 Hz. The measurements were carried out using a self-made self-oscillator and an industrial frequency meter. Resonators ZQ1 and ZQ12 for parallel circuits are selected from other batches of crystals with frequencies respectively below and above the fundamental frequency of the filter by about 1 kHz.

The filter is assembled on a printed circuit board made of double-sided foil fiberglass 1 mm thick (Fig. 2).

The top layer of metallization is used as a common wire. The holes on the side of the resonator installation are countersunk. The cases of all quartz resonators are connected to a common wire by soldering. Before parts are installed, the filter PCB is soldered into a tin-plated box with two removable covers. Also, on the side of the printed conductors, a screen-partition is soldered, passing between the leads of the resonators along the central axial line of the board. On fig. 3 shows the wiring diagram of the filter. All capacitors in the filter are KD and KM.

After the filter was made, the question arose: how to measure its frequency response with maximum resolution at home? A home computer was used with subsequent verification of the measurement results by plotting the frequency response of the filter by points using a selective microvoltmeter.

In order to view the frequency response of the filter at -100 dB, the generator must have a side noise level below the specified value, and the detector must have good linearity with a maximum dynamic range of at least 90 ... 100 dB. For this reason, the noise generator was replaced by a conventional sweep generator (Fig. 4).

(click to enlarge)

The circuit of a quartz oscillator [4] is taken as a basis, in which the relative power spectral density of the noise is - 165 dB / Hz. This means that the noise power of the generator at a detuning of 10 kHz in a band of 3 kHz is less than the power of the main oscillation of the generator by 135 dB!

The source code has been slightly modified. So instead of bipolar transistors, field-effect transistors are used, and a circuit consisting of an inductor L1 and varicaps VD1 - VD2 is connected in series with a quartz resonator ZQ5. The oscillator frequency is tuned relative to the quartz frequency within 5 kHz, which is quite enough to measure the frequency response of a narrow-band filter. The quartz resonator in the generator is similar to the filter one. In the sweep frequency generator mode, the control voltage to the varicaps VD2 - VD5 is supplied from a sawtooth voltage generator made on a single-junction transistor VT2 with a current generator on VT1. For manual tuning of the generator frequency, a multi-turn resistor R11 is used. Chip DA1 works as a voltage amplifier.

The originally conceived sinusoidal control voltage had to be abandoned due to the uneven speed of the passage of the MCF in different sections of the frequency response of the filter, and in order to achieve maximum resolution, the generator frequency was reduced to 0,3 Hz.

Switch SA1 selects the frequency of the "saw" generator - 10 or 0,3 Hz. The frequency deviation of the GKCH is set by a tuning resistor R10.

The schematic diagram of the detector block is shown in fig. 5. The signal from the output of the quartz filter is applied to the X2 input if the L1C1C2 circuit is used as a filter load. If measurements are carried out on filters loaded with active resistance, this circuit is not needed. Then the signal from the load resistor is applied to the X1 input, and the conductor connecting the X1 input to the circuit is removed on the detector's printed circuit board.

(click to enlarge)

A source follower with a dynamic range of more than 90 dB on a powerful field-effect transistor VT1 matches the load resistance of the filter and the input impedance of the mixer. The detector is made according to the scheme of a passive balanced mixer based on field-effect transistors VT2, VT3 and has a dynamic range of more than 93 dB. The combined gates of the transistors through the P-circuits C17L2C20 and C19L3C21 receive anti-phase sinusoidal voltages of 3 ... 4 V (rms) from the reference oscillator. The reference oscillator of the detector, made on the DD1 chip, has a quartz resonator with a frequency of 8,862 MHz.

The low-frequency signal formed at the output of the mixer is amplified by about 20 times by an amplifier on the DA1 chip. Since the sound cards of personal computers have a relatively low-impedance input, a powerful K157UD1 op amp is installed in the detector. The amplifier frequency response has been adjusted so that below 1 kHz and above 20 kHz there is a gain rolloff of approximately -6 dB per octave.

The oscillator is mounted on a printed circuit board made of double-sided foil fiberglass (Fig. 6). The top layer of the board serves as a common wire, the holes for the leads of the parts that do not have contact with it are countersunk. The board is soldered in a 40 mm high box with two removable covers. The box is made of tin plate.

Inductors L1, L2, L3 are wound on standard frames with a diameter of 6,5 mm with trimmers made of carbonyl iron and placed in screens. L1 contains 40 turns of PEV-2 0,21 wire, L3 and L2 - 27 and 2+4 turns of PELSHO-0,31 wire, respectively. Coil L2 is wound on top of L3 closer to the "cold" end. All chokes are standard - DM 0,1 68 μH. Fixed resistors MLT, tuned R6, R8 and R10 type SPZ-38. Multi-turn resistor - PPML. Permanent capacitors - KM, KLS, KT, oxide - K50-35, K53-1.

The establishment of the GKCH begins with setting the maximum signal at the output of the sawtooth voltage generator. By controlling the signal at pin 6 of the DA1 chip with an oscilloscope, the trimming resistors R8 (gain) and R6 (offset) set the amplitude and shape of the signal shown on the diagram at point A.

By selecting the resistor R12, stable generation is achieved without entering the signal limiting mode. By selecting the capacitance of the capacitor C14 and adjusting the L2L3 circuit, the output oscillatory system is tuned to resonance, which guarantees a good load capacity of the generator. The L1 coil trimmer sets the oscillator tuning limits within 8,8586-8,8686 MHz, which marginally covers the frequency response band of the tested quartz filter. To ensure the maximum tuning of the GKCH (at least 10 kHz) around the connection point L1, VD4, VD5, the top layer of the foil is removed. Without load, the output sinusoidal voltage of the generator is 1 V (rms).

The detector unit is made on a printed circuit board made of double-sided foil-coated fiberglass (Fig. 7). The top layer of foil is used as the common wire. Holes for the conclusions of parts that do not have contact with a common wire are countersinked. The board is soldered in a tin box 35 mm high with removable covers. Its resolution depends on the quality of manufacture of the attachment.

Coils L1-L4 contain 32 turns of wire PEV-0,21, wound round to round on frames with a diameter of 6 mm. Trimmers in coils from armor cores SB-12a. All chokes type DM-0,1. Inductance L5 - 16 μH, L6, L8 - 68 μH, L7 - 40 μH. Transformer T1 is wound on an annular ferrite magnetic circuit 1000NN of size K10x6x3 mm and contains 7 turns in the primary winding, 2x13 turns of PEV-0,31 wire in the secondary.

All tuning resistors - SPZ-38.

During the pre-tuning of the block, a high-frequency oscilloscope controls the sinusoidal signal at the gates of transistors VT2, VT3 and, if necessary, adjusts the coils L2, L3. Trimmer coil L4 the frequency of the reference oscillator is removed below the filter bandwidth by 5 kHz. This is done in order to reduce the number of various interferences that reduce the resolution of the device in the working area of ​​the spectrum analyzer.

The oscillator is connected to a quartz filter through a matching oscillatory circuit with a capacitive divider (Fig. 8).

During tuning, this will allow you to get low attenuation and ripple in the passband of the filter. The second matching oscillatory circuit, as already mentioned, is located in the detector attachment. After assembling the measurement circuit and connecting the output of the set-top box (connector X3) to the microphone or line input of the sound card of a personal computer, we launch the spectrum analyzer program. There are several such programs. The author used the program SpectraLab v.4.32.16, located at: cityradio.narod.ru/utilJties.html. The program is easy to use and has great features.

So, we launch the "SpektroLab" program and, by adjusting the frequencies of the GKCH (in manual control mode) and the reference oscillator in the detector attachment, we set the peak of the GKCh spectrogram to around 5 kHz. Further, by balancing the mixer of the detector attachment, the peak of the second harmonic is reduced to the noise level. After that, the GKCh mode is turned on and the long-awaited frequency response of the filter under test appears on the monitor. First, the swing frequency of 10 Hz is turned on and, by adjusting the center frequency using R11, and then the swing band R10 (Fig. 4), we set an acceptable "picture" of the filter's frequency response in real time. During measurements, by adjusting the matching circuits, minimal passband ripple is achieved. Further, to achieve the maximum resolution of the device, we turn on the swing frequency of 0,3 Hz and set the maximum possible number of Fourier transform points (FFT, author 4096..8192) and the minimum value of the averaging parameter (Averaging, author 1) in the program. Since the characteristic is drawn in several passes of the GKCh, the storage peak voltmeter mode (Hold) is switched on. As a result, on the monitor we get the frequency response of the filter under study. Using the mouse cursor, we obtain the necessary digital values ​​of the obtained frequency response at the required levels. In this case, one must not forget to measure the frequency of the reference oscillator in the detector attachment, in order to then obtain the true values ​​of the frequencies of the frequency response points.

After evaluating the initial "picture", the frequencies of the series resonance ZQ1n ZQ12 are adjusted, respectively, to the lower and upper slopes of the filter's frequency response, achieving a maximum squareness of -90 dB. In conclusion, using the printer, we get a full-fledged "document" for the manufactured filter. As an example, in fig. 9 shows the spectrogram of the frequency response of this filter. The spectrogram of the GKCH signal is also shown there. The visible unevenness of the left slope of the frequency response at the level of -3 ... -5 dB is eliminated by rearranging the ZQ2-ZQ11 quartz resonators.

As a result, we obtain the following filter characteristics: -6 dB passband - 2,586 kHz, frequency response unevenness in the passband - less than 2 dB, squareness factor -6 / -60 dB levels - 1,41; by levels -6/-80 dB - 1,59 and by levels -6/-90 dB - 1,67; attenuation in the band - less than 3 dB, and behind the band - more than 90 dB.

The author decided to check the results obtained and measured the frequency response of the quartz filter point by point. For measurements, a selective microvoltmeter with a good attenuator was required, which was a microvoltmeter of the HMV-4 type (Poland) with a nominal sensitivity of 0,5 μV (at the same time, it well fixes signals with a level of 0.05 μV) and an attenuator of 100 dB.

For this measurement option, the scheme shown in Fig. 10 was assembled. XNUMX. Matching circuits at the input and output of the filter are carefully shielded. Shielded connecting wires are of good quality. The "ground" circuits are also carefully made.

By smoothly changing the frequency of the GKCH with resistor R11 and switching the attenuator by 10 dB, we take the readings of the microvoltmeter, passing through the entire frequency response of the filter. Using the measurement data and the same scale, we build a graph of the frequency response (Fig. 11).

Due to the high sensitivity of the microvoltmeter and the low side noise of the GKCH, signals at the level of -120 dB are well fixed, which is clearly reflected in the graph.

The measurement results were as follows: -6 dB bandwidth - 2,64 kHz; uneven frequency response - less than 2 dB; -6/-60 dB squareness ratio is 1,386; by levels -6/-80 dB - 1,56; by levels -6/-90 dB - 1,682; by levels -6/-100 dB - 1,864; attenuation in the band - less than 3 dB, behind the band - more than 100 dB.

Some differences between the measurement results and the computer version are explained by the presence of accumulating digital-to-analogue conversion errors when the analyzed signal changes in a large dynamic range.

It should be noted that the above graphs of the frequency response of the quartz filter were obtained with a minimum amount of tuning work and with a more careful selection of components, the filter characteristics can be noticeably improved.

The proposed oscillator circuit can be successfully used to measure single-signal selectivity, as well as to measure the dynamic range of transceivers up to 110...120 dB.

This device can be successfully used to evaluate the quality indicators of the IF path of transceivers, the operation of AGC and detectors. By applying the signal of the oscillator to the detector, at the output of the set-top box to the PC we get the signal of the low-frequency oscillator of the oscillating frequency, with which you can easily and quickly adjust any filter and cascade of the low-frequency path of the transceiver.

It is no less interesting to use the proposed detector attachment as part of the panoramic indicator of the transceiver. To do this, connect a quartz filter with a bandwidth of 8...10 kHz to the output of the first mixer. Further, the received signal is amplified and applied to the input of the detector. In this case, you can observe the signals of your correspondents with levels from 5 to 9 points with good resolution.

Literature

1. Usov V. SSB quartz filter. - Radio amateur, 1992, No. 6, p. 39,40.
2. Drozdov VV Amateur KB transceivers. - M.: Radio and communication, 1988.
3. Klaus Reban (DG2XK). Optimizierung von Egenbau-Quarzfiltern rnit der PC-Soundkarte. - Funkamateur, No. 11,2001, 1246, S. 1249-XNUMX.
4. Frank Silva. Shrnutzeffekte vermeiden und beseitig. - FUNK, 1999.11. S. 38.

Author: G.Bragin (RZ4HK)

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