ON6MU (ex ON1DHT)

70 MHz converter
RE-RXC70/10

SA602/NE602 based converter for 4-meterband to 10-meterband
Receive singals from the four meterband on your shortwave receiver!

About the 4-meterband converter RXC70/10:

This is a very sensitive 70Mc converter allowing you to receive the band (70Mc...72Mc) on your general coverage receiver (28Mc...30Mc). It receives all types of modulated transmissions. It all depends on the receiver used. I've tested this project on a allmode Yaesu FRG-100 receiver. Within certain limits you can change the output frequency to suit your needs. The converter is very stable, low noise, sensitive and low on power consumption.
The heart of the converter has been built around Philips SA602 (NE602 or NE612), a twice balanced mixer oscillator. This IC finds his applications in layer capacity communication systems, cellular radio applications, RF data left, VHF-transceivers, broadband LAN's ed. IC in a ordinary 8-pin dual-in-line can be bought implementation (DIP) or 8-pin SO (surface-mount miniature package) implementation. Both implementation has a low cost. SA/NE602 a very low usage of only 2,4mA has! The total usage of the converter amounts to only 15mA. Therefore also uncomplicated usable applications fed with battery.

The SA602A is a low-power VHF monolithic double-balanced mixer with input amplifier, on-board oscillator, and voltage regulator. It is intended for high performance, low power communication systems. The guaranteed parameters of the SA602A make this device particularly well suited for cellular radio applications. The mixer is a “Gilbert cell” multiplier configuration which typically provides 18dB of gain at 45MHz. The oscillator will operate to 200MHz. It can be configured as a crystal oscillator, a tuned tank oscillator, or a buffer
for an external LO. For higher frequencies the LO input may be externally driven. The noise figure at 45MHz is typically less than 5dB. The gain, intercept performance, low-power and noise characteristics make the SA602A a superior choice for high-performance battery operated equipment. It is available in an 8-lead dual in-line plastic package and an 8-lead SO (surface-mount miniature package).

70 MHz converter technical specifications

RXC70/10 SCHEMATIC

Homemade 70 Mc converter project: schematic of a SA602/NE602 based converter for 4-meterband to 10-meterband

PARTS

IC1 = NE602, NE612, SA602A, SA612A
IC2 = 78L06
T1 = BC338
C1 = 10uF/25v
C2 = 100nF
C3 = 100nF
C4 = 10uF/25v
C5 = 47uF/16v (tantaal)
C6 = 3n3
C7 = 33pF
C8 = 22pF
C9 = 0...22pF (green)
C10 = 680pF
C11= 4.7nF
C12 = 0...40pF (white)
C13 = 47pF (poly)
C14 = 39pF (poly)
C15 = 47nF
C16 = 330pF
C17 = 330pF
C18 = 100pF*
C19 = 470pF (you can add in serie with C19 a 100 Ohm potmeter to regulate the output voltage)
C20 = 333pF
R1 = 1k
R2 = 2k2
R3 = 100 Ohm
R4 = 5k6
R5 = 1k2

Coil specifications:
L1 = 6 wnd 1mm silver coated and spaced 1mm, 9mm coildiameter (inside diameter 7mm), tap on 1,5 wnd from the cold end (cold end = from the ground).
L2 = 10 wnd 0,4mm email, 5mm coildiameter (7mm long, inside diameter 4mm) +/-0,1uH. Can be tweaked if needed to change the bandpass range (or make C18 a variable capacitor).
L3 = shokes (RFC) 10uH +/- or use a ferite bead

 

Highlighted

SA602/NE602 based converter for 6-meterband to 10-meterband

The converter explained

The heart of the converter has been built around Philips SA602 (NE602), a double balanced mixer oscillator. This IC finds his applications in layer capacity communication systems, cellular radio applications, RF data left, VHF-transceivers, broadband LAN's ed. IC in a ordinary 8-pin dual-in-line can be bought implementation (DIP) or 8-pin SO (surface-mount miniature package) implementation. Both implementation has a low cost. SA/NE602 a very low usage of only 2,4mA has! The total usage of the converter amounts to only 15mA. Therefore also uncomplicated usable applications fed with battery.
The mixer is Gilbert cell tip quadrant configuration which 18dB can provide conversion gain. The built in Local oscillator work to maximum 200MHz tank oscillator coordinated with a high Q or crystal oscillator. The highest frequency which we can bring to the input of this IC amounts to 500MHz.
In this project we apply a crystal retrieve oscillator. Frequency stability is excellent and depends mostly of the surroundings temperature crystal then the IC itself. As it happens, a very ingenious and efficiently temperature compensating bias is built in. Important to know is that the oscillator already has an internal bias and therefore don't need extra dc-bias. Only at very high frequencies a raised direct current can be necessary. This one remedies by placing between the mass and resistor at pin 7 of a value of of 22k.
The NE602 LO works up to 200MHz and the input up to 500MHz, therefore a huge 'reserve' is available since we use a much lower LO input frequency. We want to convert, as it happens, 70MHz to 28MHz. This means therefore that we must mix with a frequency of 42MHz, meaning 70MHz - 42MHz = 28MHz output. To allow the converter to be calibrated to obtain the exact frequency, a regulable condenser of 40pF (C7) is added to the oscillator. With this you can vary the termination frequency of the converter +-300 Hz.
Without much adapting you can also use the more currently available 40MHz crystal, but then the termination frequency of the converter will be 30MHz ipv 28Mhz (40MHz LO + 30MHz OUT = 70MHz IN).
There are not so many receivers that go byond 30Mc but most transceivers will receive way beyond 30Mhz.
The Gilbert cell is a differential amplifier which has balanced cell feeds. The differential gives extra gain and stipulate the noise number as well as the strong indicator behaviour of the recipient/converter. And these processes values up to -199dBm with 12dB S/N ratio. The symmetrical RF input (pin 1 and 2) has internal bias, thus we avoid external DC bias (to see C10 and C15)! THE RF input amount to capacitantie 3pF. There we connect single-ended coordinated LC-kring with parallel a resonance a frequency of 70MHz. I used a balanced input for attenuation of second-order products. The tank circuit can peaked to best reception with C9. This is done best on a frequency where we want best sensitivity, for example 70,300 MHz. To start, move C9 in the middle position. When we have wound the coil L1 correctly, C9 does not need much to be adjusted. If there is no station to tune in to, then regulate C9 till you hear maximum noise.
To have a 50 ohm input by means of C7 and a tap at 1.5 turns from the cold end of the coil. Of course you'll need on 70MHz tuned antenna too HI.
The sensitivity of the converter amounts to 0.22uV at 12dB SINAD. Third-order the intercept point is -13dBm. This is approximately +5dBm output interception because of the RF gain.
The mixer has an internal DC-bias, by means of we connected the output (pin 4 and 5) with a 1k5 resitor to Vcc. Disengaging of the bias happens by means of C16, since we exploit here only a single termination instead of a balanced output. A balanced output will improve something, but to keep the schematic diagram simple, I have not applied this.
To allow only the 28Mc signal to pass through to T1 and into your radio I added a bandpass filter made out of C16,C17,L2 and C18. How crazy it may sound it actually improved the gain too.
The termination capacitance of the mixer (pin 5) amounts to 1.5kOhm. Given the termination indicator and the RF output voltage is a bit on the low side to connect directly to the recipient (radio), there is a amplified step to added which exists from a single BC547 transistor and als serves s a buffer between your receiver and NE602.
An signal/noise ratio improvement can be made by using dual-gate mosfet ipv BC338 (or equivalent). The noise number of SA/NE602 is 4,6dB at 20°C and T1 ads its own noise level to it, as a result we end up with an average noise number of approximately 5dB.

Bandpass filters out all unwanted frequencies from the mixer
C18 and L2 acts like a bandpass in this schematic. It passes signals approx. 26...30MHz. If using another LO frequency it could be needed to tweak C18. So it isn't a bad idea to uses a variable capacitor (trimmer) to fine tune the bandpass in this case.

More about the SA602 (NE602, SA612) in this project

The SA602A is a Gilbert cell, an oscillator/buffer, and a temperature compensated bias network as shown in the equivalent circuit. The Gilbert cell is a differential amplifier (Pins 1 and 2) which drives a balanced switching cell. The differential input stage provides gain and determines the noise figure and signal handling performance of the system.
The SA602A is designed for optimum low power performance. When used with the SA604 as a 45MHz cellular radio second IF and demodulator, the SA602A is capable of receiving -119dBm signals with a 12dB S/N ratio. Third-order intercept is typically -13dBm (that is approximately +5dBm output intercept because of the RF gain).
Besides excellent low power performance well into VHF, the SA602A is designed to be flexible. The input, RF mixer output and oscillator ports can support a variety of configurations provided the designer understands certain constraints, which will be explained here.
The RF inputs (Pins 1 and 2) are biased internally. They are symmetrical. The equivalent AC input impedance is approximately 1.5k || 3pF through 70MHz. Pins 1 and 2 can be used
interchangeably, but they should not be DC biased externally.
The oscillator is capable of sustaining oscillation beyond 200MHz in crystal or tuned tank configurations. The upper limit of operation is determined by tank “Q” and required drive levels. The higher the “Q” of the tank or the smaller the required drive, the higher the permissible oscillation frequency. If the required LO is beyond oscillation limits, or the system calls for an external LO, the external signal can be injected at Pin 6 through a DC blocking capacitor.
External LO should be at least 200mVP-P. It is important to buffer the output of this circuit to assure that switching spikes from the first counter or prescaler do not end up in the oscillator spectrum. The dual-gate MOSFET provides optimum isolation with low current. The FET offers good isolation, simplicity, and low current, while the bipolar transistors provide the simple solution for non-critical applications. The resistive divider in the emitter-follower circuit should be chosen to provide the minimum input signal which will assure correct system operation.

Notes:
Tune to the desired bandpass frequency (70Mc) with C9 until you have the best reception.
Use C12 to calibrate the output frequency to your receiver. The output frequency can be adjusted up to 300Hz.

Other output frequencies can be set by changing the 42MHz Xtal: Example: output frequency is 28 MHz then you use a 42 MHz Xtal (50MHz - 42MHz = 28MHz).
Build the converter in a metal box and use small connections between the parts.

Important: use only a antenna designed for 70MHz! A simple dipole of around 2 meters in length (two times 96 cm) will work just fine if the propagation is there.
A quater wave antenna or a J-pole can easily be made. Have a look at my
1/2 lambda vertical antenna.

PCB:
A PCB has not been designed for this project yet. Any volunteers out there?


 

Afdrukken van dit project

More about his:

SA602/NE602 based converter for 6-meterband to 10-meterband

 

Please let me know if you build it and want your pictures on this webpage.
Thanks in advance.

Please take also a look at my 50Mc converter


Note: if you want to commercialise, publish or distribute this project
then you need to ask permission to do so.


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