Frequency counter 1.3 GHz                   

Last revision 06/2011

Technical characteristics

 Field of measure

 300 Hz - 50 Khz with resolution 1 Hz or 10 Hz

 50 Khz - 52 Mhz with resolution 1 Hz or 10 Hz

 30 Mhz - 1,3 Ghz with resolution 10 Hz or 100 Hz

 Sensibility

  300 Hz - 1 Khz >= -20dBm/ 600 Ω (approximately 77mV)

   1 Khz - 10 Khz >= -35 dBm /600 Ω (approximately 13 mV)

 10 Khz  -  100 Khz >= -40dBm /600 Ω (approximately 8 mV)

 100 Khz  -  40 Mhz >= -28 dBm/50 Ω (approximately 9 mV)

   40 Mhz  -   50 Mhz >= -25 dBm/50 Ω (approximately 13 mV)

   50 Mhz - 1100 Mhz >= -26 dBm/50 Ω (approximately 11 mV)

 Indication on the display

Measure type

 NOT IF

 IF - VFO

 VFO - IF

 IF + VFO

Range of measure

 50K

 50M

 1G3

Resolution

   1 Hz

 10 Hz

 100Hz

Counting and transfer time (GATE)

 Two asterisks, one during the counting time and the other during the transfer time, are displayed

Power requirements

 without back lighting system, 9 V inner battery, duration ≈ 50 h

 with back lighting system, external power source from 9 to 13 Vcc (200mA) or 9 Vac

 Dimensions mm 142x64x35 (inc.5.6x2.5X1.2)

Electrical diagram

Figure 1 is the electrical diagram, realized with the famous freeware SW (http://www.holophase.com/) for schematics and PCB; the CIRCAD, version 4.20t. In the section download, you find the original.

Fig. 1

All it is composed from little pieces. Leaving from connector BNC1 we find the diode D1 and D2 in anti parallel, they clip strong signals that could damage Q1 and U1. The Q1 transistor, and relates components, constitute a preamplifier for the frequencies until 52 MHz. In order to improve the sensibility and the impedance adaptation for frequencies over 100Mhz, I have inserted two ferrite bead (Amidon FB 43 101) on the terminals of C2 (L2 and L3 of the schematics). Over 100 MHz the reactance of L2 and L3 it is very high and the signal sees only the 50 Ω of the U664; the small group R4 C6 is a filter for Low Frequencies, that serves in order to improve the ability to measure until to 50 KHz. I had verified that with some generators of LF, the measurements of weak signals, less than 10 - 20 KHz they ware much difficult or impossible, especially if the output impedance of the generator was higher then 50Ω. I had partially resolved this problem with the filter R4 C6 and the ferrite bead on the emitter of Q2, sacrificing the ability to measure beyond the 30 MHz. In this version I have preferred to extend the ability to measure without prescaler until to 52Mhz and putting a switch (SW5) FILTER that inserts the filter, if necessary, and with effect only in the 52Mhz band, moreover I have changed the value of C6 to drastically limit the band. If the filter is switched on, on the display it is visualized the new limit of band. The presence of the BF filter has allowed me also to extend the measure ability low. The value of C2 is passed from 22nF 100nF and now values of 300Hz at -20 dBm can be measured.

Through a RL1 exchange, the amplified signal passes on the base of Q2 that constitutes the squarer amplifier, L1 serves for increase the gain to the higher frequencies.

The signal to be measured, amplified from Q1 and squared from Q2, it is applied to the input of the PIC through R8. This resistor limits the current on the ports of the PIC and at the same time avoids that the collector of Q2 is connected to ground, when the PIC puts PORTS RA3 (pin 2) and RB6 (pin 12) to 0.

Returning on BNC1, through the C1 capacitor, the signal to measure is applied also to the input of the IC U1 that is the U664. This integrated, been born like prescaler for the synthesizer of the TV and therefore easy available, incorporates inside a preamplifier and a divisor for 64 able ones to operate from 30 MHz until to 1.3 GHz. The output of U1 is a squared wave of approximately 1Vpp, with frequency that varies from 0.468 MHz to 20.3 MHz (30 Mhz/64 and 1.3 Ghz/64).

When RL1 is fed, the output of U1 is connected to the input of Q2, amplified a second time and sended to the PIC for being counted.

RL1 is a 5V micro relay connected directly to the PIC. As the relay I used in the prototype, it is practically unavailable, I have modified the circuit in order to use that one of the NAIS model TQ2-5V; for this I have had to add the R14 resistor ( 330 Ω ) and the diode zener D4 (5.1V). Since the relay it is fed directly from the PIC, I have replaced the sliding double switch of the old version with a push-button (SW4) called BND (band) that, when pressed, it says to the PIC to change band, that is to pass from 52Mhz to 1.3Ghz or vice versa, and writing on display the value of the inserted band.

In order to save current and to lengthen the life of the battery, with the other exchange of the relay I have removed tension to U1, when Q1 is used and vice versa.

In position 50 MHz we will have:

 SW4 is opened

 RL1 is not fed

 Q1 is fed

 to the PIC the output of Q1 - Q2 arrives

 the PIC counts directly

 U1 is without feeding

In position 1.3 GHz we will have:

 SW4 is closed

 RL1 is attracted

 U1 is Fed

 to the PIC the output of U1 - Q2 arrives

 the PIC, whose  pin 1 has been connected to ground when it has been pushed SW4 (BAND), it multiplies for 64 the one which counts

 Q1 is without feeding

U2 is a small type voltage regulator (78L05); D3 serves to protect from possible reversals of polarity of the external power source and eventually to rectify the Vac, in case you decide of feeding all with a simple little transformer with a secondary of 9 - 10 Vac. The groups Z1 C8, Z2 C11, Z3 C13 serve for decoupling the feedings.

R6 is the 2 W resistor that limits the back lighting system current of the display when it is used an external power source, in this case it cannot be replaced with a diode because for the back lighting system it is not used 5 V  but directly the 9 -12V. The power dissipated from this resistor depends on the feeding voltage and in order to avoid that it dissipates too much (sees PRACTICAL REALIZATION) it would be opportune to use an external local power source from 9 - 10 Vcc. About of external power source, pay attention to those of low power, because generally they are not stabilized and some of them, without or with little load, have much highest tensions then the target.

The PIC 16F648A

This PIC constitutes the true frequency counter, or the object that counts how many cycles arrives to its input (pin 2 and 12) in a fixed and precise time interval, it elaborates the measured data and it sends the result to the display that transforms it, in numbers and letters, comprehensible from mankind. As reference for the time interval the PIC uses the 4 MHz quartz oscillator that is sufficiently stable (fixed) for the class of this instrument, while in order to render it precise, it has been inserted variable capacitor CV1, that then we will see as it must be adjusted.

In the receivers who do not have a digital frequency display, in order to know the reception frequency it is used to measure the frequency of the local oscillator and, knowing the value of the IF, that one of reception is estimated. This frequency counter, like others, executes these calculations automatically. R10 SW3 ( INC of the keyboard) and R11 SW2 (IF of the keyboard) serves to indicate to the PIC the eventual value of IF and how count it, in order going back to the frequency received from the receiver.

R13 SW4, like already said, serves in order to inform the PIC that must to insert the prescaler that divides for 64 and that therefore it must to multiply for 64 the values read to its input.

R12 C12 is a delay circuit that serves to reset the PIC when switched on.

The variations regarding the previous version are:

 PIC 16F648A  pin to pin compatible with the 16F628A but having double quantity of memory

 Use of TMR1 rather than of TMR0 for having one greater ability to count and therefore precise measures. Now the input of signal to measure is on pin 2 and 12.

 Elimination of the bead of ferrite on the emitter of Q2, adding of the beads of ferrite on the terminals of C2, adding of trimmer the VR2 and modification of the value of R5 from 22KΩ to 18KΩ. Since the circuit has revealed much stable, I have removed the bead that reduced the sensibility for the frequencies over the 20 MHz. On purpose of sensibility, keep in mind that you could only partially find various values from that I have indicated, being Q2 with low feedback, the gain and the working point of this amplifier depends  from β (the coefficient of amplification) of the transistor that, like well know , varies from each one. To reduce this problem I have inserted VR2, varying which is possible to optimize the sensibility (see calibration section). If in order to read 50 MHz, after to have regulated VR2, they had to be necessary values higher than -20 dBm tried to change the transistor.

 Substitution of the LED D6 blinking  and of the value of the R9 resistor. In order to save current I had used a blinking led, but turned out annoying. I have replaced it with a 2.5 millimetres LED of and I have reduced its current changing R9 to 560 Ω.

Practical realization

In figure 2 you see the inside of the frequency counter

Fig. 2

All it has been realized on two printed circuits (PCB) inserted inside of one plastic box of the TEKO (model 10008.9). On the right you see the smaller of the two PCB that constitute the keyboard, while largest, (97 x 58 millimetres) partially are covered from the display.

Noticed low switch (SW5) that inserts filter BF and near the trimmer (VR2) that it serves to regulating the working point and the sensibility of Q2. On the right the keyboard that is changed from the previous version and is connected with only 5 wires rather than 6 like before.

Up in the photo, noticed that I do not have inserted the 2W resistor R6 for the back lighting system. Even if on the printed circuit it is present the place and the serigraphy for this resistor, I council you not to mount it because the heat generated from the resistor and from the led of the display provokes a heating of the 4 MHz quartz with consequent loss of stability of the measures. On the other hand the back lighting system is completely useless, because in the normal conditions of lighting system of the rooms, it is not visible, then if the electrical power net is available for feeding frequency counter and all the rest, it will be also in order to illuminate the rooms.

In figure 3 you see the same PCB without the display.

Fig. 3

On the right found the R14 resistor 330 Ω ½ W and the diode Zener D4. The relay RL1 this time is of gray colour. Also in this version the PCB is fixed on the plastic box with only 3 screws, the fourth would be due to be under to BNC connector.

The up side yellow component is the socket of external feeding P1 with near the power switch SW1. To the centre we find the PIC, at low, from left side, looks connector BNC1 PCB type, the U1 prescaler and the relay RL1.

As you can notice, the only wires are those of the battery and the 5 wires flat cable that connect the keyboard.

Figure 4 it is the solder side art work and the photo of the PCB.

Fig. 4

Figure 5 is the photos of the PCB component side. For L1 I have previewed the double holes, therefore it will be possible to mount the rectangular inductor like Neosid or 6mm cylindrical inductor. Look at wire jumper connection to the right of the PIC and R8 that has a terminal longer (both partially under to Z1). The C12 capacitor is inserted inside of the socket of the PIC and therefore you will have to use one socket with a window to the centre (nearly all are therefore).

Fig. 5

Go to Part 2.