7R3357 "Fujimino40", an AGC-less Superhet Receiver for 7MHz CW

7R3357 board

First written on 10 May, 2004
Revised on 24 May, 2006



Once upon a time, there was an FM IF subsystem IC called MC3357. The Motorola's original has been obsoleted for a long time, however, New Japan Radio's NJM3357 is still in production.
The IF section of MC/NJM3357 consists of five stage cascaded differential amplifiers and thus it works as an IF limiter. The objective of this page is to propose a CW receiver featuring AGC-less IF section.

The most important point of adopting 3357 is that the LO node of the demodulating Gilbert mixer is NOT connected internally to the IF output. This means that we can use it as a product detector by applying an external BFO.
On the other hand, later devices such as 3359, 3362, etc. are NO GOOD for demodulating CW signals because of internally connection between IF output and demodulator LO input.

Now, 7R3357(R) has a nickname "Fujimino40." The name comes from the place where the main manufacturing site of New Japan Radio locates.

Circuit description

7R3357 schematic
Fig.1 Schematic of 7R3357R (click figure to enlarge).

The schematic of 7R3357R is shown in Fig.1.
The last letter "R" indicates that the receiver has a remote control capability for transceive operation. The pair transmitter, 7T9521 is here.

The RF section includes an RF attenuator (1kΩ VR), a diode limiter pair and a narrow band (approx. 150kHz bandwidth) BPF.

The VFO is varactor tuned and covers 7.442 to 7.472MHz (IF is 442kHz due to the BPF response described below).
A 10k-ohm resistor between pin 2 and GND is required to ensure the LO oscillation.

The MIX output is led to the IF BPF, "Cerladder Filter." The Cerladder Filter is a ladder BPF using ceramic resonators.
Resonant frequency of each resonator should be matched within 100Hz (less than 1/10 of the filter bandwidth) to obtain the best result. I have selected 3 resonators from 10 samples by measuring C-MOS oscillator frequency.
Component values for 1kHz BW filter are given by Mr.Takahiro KATO, JA9TTT/1. My Cerladder BPF responses are shown in later section.
The AGC-less feature is achieved by adopting both the Cerladder BPF and the high gain IF limiting amplifier.

The optimum BFO injection level ranges from 10 to 15 mVrms.
Larger than 20mV BFO results S/N degradation and parasitic oscillation. This is the most different point from other Gilbert mixers.
In Fig.1, the emitter resistor of the BFO is divided to configure a simple attenuator.

Demodulated audio signal is led to the 2nd order active LPF (approx. 1.5kHz cutoff) made of internal operational amplifier.

LM386 (NJM386B in actual receiver) is adopted as audio amplifier. Pins 1 and 8 are left open, thus the gain is fixed to 26dB.
A 2.2kΩ input shunt resistor reduces thermal noise due to the internal bias resistor.

For transceive operation, pins 3 and 12 of MC/NJM3357 are toggled from VCC to GND. Pin 12 triggers internal audio mute transistor.
A low-power relay such as OMRON G6S or FUJITSU SY-12 is recommended.


breadboard front view rear view
(a) Breadboard (b) Front view (c) Rear view
Fig.2 Implementation of my 7R3357R.

Figure 2 shows my implementation.

The whole circuit is built on a 72 x 95 mm breadboard.
No special consideration is required, however, use of NP0 capacitors and trimmers around RF BPF, VFO and BFO is recommended.
VFO inductor (L3) should be bonded on the breadboard. I use nail polish as adhesive agent.
Remote control relay should be separated from the VFO inductor to avoid magnetic induction.

The dimension of the receiver enclosure is 150 x 100 x 40 mm. There is enough space for six AA cells (a 6F22 battery is on the photographs instead).
A phone jack, audio volume, tuning and RF attenuator VR's, power switch and a red-and-green LED are located on the front panel.
A mini-DIN (6-pins) connector for remote control, antenna and speaker jacks are on the rear panel.


The measured sensitivity is -10dBuV under 10dB (S+N+D)/(N+D) condition.
The selectivity of the Cerladder filter is shown in Fig.3.

Fig.3 Measured “Cerladder filter” response.
(a) out of band response(b) overall response(c) passband response


I greatly thank OM Takahiro KATO, JA9TTT/1, for his helpful discussion on the Cerladder filter.


Note: Following references are in Japanese.
  1. Takahiro KATO, JA9TTT, CQ ham radio, Jan. 2006 to June 2006 issues, CQ Publishing, Tokyo

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