Remote probe - frequency divider by 10 for frequency counter FC250. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The operation of the FC250 frequency meter with an external probe (VSH) transmitting a signal at the same frequency revealed instability of its readings and overheating of the DD2 chip on the FC250 frequency meter board [1] at frequencies above 150 MHz. This frequency is the limit for most microcircuits used in pre-amplifiers-shapers (PA) and at the FC250 input. Therefore, a new VSC was manufactured, the circuit of which with the PU is shown in Fig. 1. PU is assembled according to the scheme of fig. 1 from [1], the values ​​of some elements have been changed. The VSC is assembled on two microcircuits: DA1 (ADCMP604KSZ-R2) - a CMOS comparator with a delay time of 1,6 ns, an input differential resistance of up to 70 kOhm, and a frequency divider by 10 - DD1 (KS193IE3) [2], which has an operating frequency range from 100 kHz to 270 MHz.

Rice. 1. Scheme of a remote probe with PU

The method of supplying a bias voltage to the inputs of the comparator DA1 using resistors R3-R7 allows the tuning resistor R3 to change the hysteresis voltage and adjust the sensitivity of the VSC. The high input impedance of the ADCMP604 comparator, reaching 70 kΩ, explains the large resistance of resistors R4 and R5, chosen so as to least shunt the comparator inputs. The outputs of the comparator DA1 are connected to the inputs of the divider DD1 without separating capacitors through terminating resistors R8-R10, which are needed in order to prevent the supply of an anti-phase voltage of more than 2 V to the inputs of the divider in static mode.

Unlike its prototype SP8690A, the KS193IE3 is not a fully ECL chip, the bias voltage at its inputs (pins 11 and 12) complies with the PECL standard, which makes it possible to connect them directly to the outputs of the ADCMP604 comparator of the LVDS standard. In this case, an anti-phase rectangular signal from the ADCMP604 is fed immediately to both differential inputs of the divider, which allows the VSC to operate in almost the entire operating frequency range of the KS193IE3.

In static mode, the voltage difference at the inputs of the KS193IE3 chip of 0,5 V prevents its self-excitation, and the supply of an anti-phase signal of the LVDS level (0,35 V) made it possible with the new VSC to obtain a range of frequencies measured by the FC250 from 400 kHz to 270 MHz with a small input capacitance, large input resistance and measurement resolution of 100 Hz. In the range from 1 to 200 MHz, the sensitivity of the FC250 frequency meter with VSH is not worse than 0,35 V, in the "soft" controlled self-excitation mode of the DA1 comparator it is not worse than 0,2 V, and at the edges of the measurement range it is not worse than 0,65 V.

There was no goal to achieve the lower limit of the operating frequency of the KS193IE3 divider at 100 kHz. But with an increase in the capacitance of capacitors C1 and C2 to 43 pF, the lower operating frequency of the VSC became less than 300 kHz.

The supply voltage +5 V is supplied to the VSC from the voltage regulator of the frequency meter FC250, the current consumption is approximately 35 mA. Pin 6 of the KS193IE3 divider, TTL open-collector output, is not used and left unconnected. From its outputs 2 and 4, the antiphase signal of the ECL standard is fed through a loop 0,3-1 m long to the inputs of the PU, which is located on the FC250 board and generates the TTL level signals necessary for the operation of the frequency meter [1, 3].

Resistor R12 is installed at the end of the loop, at the point of its connection with the PU. Both differential inputs of the VSC are equivalent, they are not connected either to a common wire or to the power line of the frequency meter.

During operation, both VSC contacts are connected to the measured object. For convenience, one of the VSC contacts can be connected to the common wire of the device being measured with a piece of wire up to 10 cm long with a crocodile clip at the end. The use of the VSC allows you to measure the frequency of TTL and ESL levels, the frequency of the local oscillators of various types of radio receivers in the ranges from LW to VHF-2 with a slight effect of the capacitance of the VSC on their frequency. On ranges with a large frequency overlap, in particular VHF-2, due to a decrease in the local oscillator voltage in the low-frequency section, the frequency can only be measured in the controlled self-excitation mode of the VSC, while connecting other probes with a lower input impedance led to generation disruption. In the case of an insufficient level of the measured signal, when the sensitivity level of the VSC is set to a low level, and in the event of poor contact between the probe and the device being measured, the frequency meter readings are underestimated or disrupted.

When a signal with a frequency of 100-200 MHz and a voltage of more than 0,5 V, which has an irregular shape, is applied to the VSC, the DA1 comparator can double the frequency. In this case, to reduce the signal voltage, the VSC is connected to the signal source through an attenuator, the details of which can be soldered directly to the contacts of the VSC. The FC250 can still measure signals from 50 Hz to 100 MHz with 10 Hz resolution. To do this, instead of a loop with a VSC, wires up to 6 cm long with limiting resistors up to 7 kOhm are connected to capacitors C1 and C20 at the input of the PU [1].

The VSC is assembled on a board made of fiberglass laminated on both sides with a thickness of 1,5 mm. The board is made by cutting through the foil after drilling holes on it. The board drawing is shown in fig. 2. PU can be assembled on the board according to fig. 2 in 1].

Rice. 2. Drawing of the remote probe board

The device uses capacitors and resistors, with the exception of R3 and R12, for surface mounting, size 1206 or 0805. Variable resistor R3 - 3310Y or any other suitable size and pinout. Resistor R12 - output, with a power of 0,125 W, is located at the end of the loop connected to the PU Comparator DA1 - for surface mounting in the SOT 323-6 package, the divider KS193IE3 (in the DIP-16 package) is installed in the panel, from which the unused contacts are removed. When installing the KS193IE3 chip directly on the board, the ends inserted into the holes are removed from its unconnected pins.

The arrangement of parts is shown in fig. 3. Jumpers on the contact pads for the wires of the loop, which prevent the foil from peeling off when they are unsoldered, and the VSC contacts are made of tinned wire with a diameter of 0,75 mm. The remaining jumpers and "firmware" of the edges of the board are made with tinned wire with a diameter of 0,5 mm. A photograph of the bottom side of the printed circuit board is shown in fig. 4. A properly assembled VSC does not require adjustment. If the self-excitation of the VSC is not eliminated by the resistor R3, then the main reason for this is a break (bad soldering) of one of the outputs of the DA1 comparator. The probe is placed in a plastic case. Mounting holes are drilled "in place" on the tinned edges of the VSC board. You can simply wrap the probe with adhesive tape, leaving the contacts and slot of the tuning resistor R3 outside.

Rice. 3. Location of parts on the VSC board

Rice. 4. Photo of the bottom side of the PCB

On fig. 5 shows an example of measuring the maximum frequency of 300 MHz.

Rice. 5. Example of measuring the maximum frequency of 300 MHz

Literature

1. Panshin A. Pre-amplifier-shaper for frequency counter FC250. - Radio, 2015, No. 2, p. 18-20.
2. Khlyupin N. Microwave frequency dividers. - URL: ra4nal.qrz.ru/prescaler.shtml.
3. Nechaev I. Probe-comparator for a frequency meter. - Radio, 2014, No. 7, p. twenty.

Author: A. Panshin

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