The well-known mixer NE612 (NXP) will be compared to an AN612 (Matsushita/Panasonic) mixer that has been unsoldered from an old CB-SSB-radio. Comparison will include output voltage level and spectroscopic analysis of a 9MHz SSB signal.
When we talk about about integrated double balanced mixers (DBM) and say the number “612” we usually talk about the NE612 (aka SA/NE/602/612 in free combination of letters and digits). This IC uses a so called “Gilbert Cell” and has been developed by Dutch manufacturer Philips (nowadays NXP) some 30 years ago.
The IC has been intended to be used in cellphone applications, is a low voltage device (6 to 7V VDD approx., 8V DC max.) and has low power consumption . Frequency range is up to 500MHz (input signal) and gain is around 12 to 15dB. It has an integrated oscillator circuit that can be used with crystals connected to PIN6.
The IC has been widely adopted by amateur radio constructors and is still available today mainly in SMD package. When we examine homemade QRP radios published on the internet e. g., in 90% of cases one or more NE602 mixers will be found in the transceivers. One real advantage of the NE612 family is that only a few external components are required for building up a relatively acceptable working rf mixer.
In my radios I usually use the NE602 and its equivalents therefore for the DSB generator circuit and the transmit mixer. For receiving purposes it can be used for the higher bands (f >= 14MHz), on the lower bands the relatively low IMD performance (IMD3 about 15dB) shows severe shortcomings particularly on the 40 meter band where strong off-band broadcaster generate high signal levels and therefore overdriving the mixer’s input stage.
Due to the low IMD performance the IC also has weaknesses when being used as a DSB generator. The following findings occured when I analyzed the spectrum of a simple DSB/SSB generator equipped with an NE602.
NE612 DSB generator circuit under test
The NE612 here has been equipped with an additional resistor network (2x56k and a var. resistor with 10k) to get better carrier suppression features. To enhance output a transformer has been added to use PINs 4 and 5 which are the output stages of the circuit.
When driven with an dual tone audio signal (the 2 frequencies not harmonically related) we get an output voltage of about 50mV pp. and the spectrum shown below:
We can observe some IMD 3 and 5 products about 30dB below peak voltage. This is an outcome a little away from what can be expected from an SSB generator.
AN612 also is a very simple mixer that has been developed by Matsushita (Japan, now Panasonic) and has been used in various types of SSB radios for the 11m-Band (CB). In contrast to NE612 it does not contain an internal oscillator.
The IC comes in a 7 lead IC case (SIP7), please refer to datasheet. The IC is manufactured still today and available from various vendors on the internet. I ordered a package of ten from a Chinese ebay seller and found the ICs worked the same way like an original one from a PRESIDENT CB radio. They actually were no fakes.
The IC has a higher VDD so that it can be connected directly to the 12V rail of a standard battery operated radio. In contrast to the NE612 there is no need for a voltage regulator. Also the whole circuit only needs 7 external components:
Performance is quite interesting. When comparing this circuit to the NE612 DSB generator, we find that the output voltage is 4 times higher than that of its namesake. It equals to 200mV pp. The output spectrum also has slightly improved concerning IMD performance:
We see a little fewer IMD products with slightly decreased signal strength.
The AN612 is a not very well known but so much the better interesting mixer IC for the ambitious radio designer who wants to build hardware defined radios. The main locations in a radio will be the DSB generator and the transmit mixer. The IC is cheap, very well available and reveals a slightly higher performance than the other “612”, the NE612. And, overall, the circuit is very simple.
Vy 73 de Peter (DK7IH)