Getting My FE-5650A Rubidium Frequency Standard Working

The Important Discovery

I discovered that my particular FE-5650A CPOM / Option-58 rubidium frequency standard requires both +15 V and +5 V power supplies.

Previously, I had been supplying only +15 V to pin 1. Although the unit heated up and drew substantial current, it was not actually being powered according to the requirements of this particular version.

Once I received the correct pinout and power requirements from the seller, I changed the wiring accordingly.

DB-9 Connections

I wired the DB-9 connector as follows:

An important point is that I joined the grounds of the two power supplies together. Pins 2 and 5 therefore share a common ground. Pin 7 provides the signal-ground reference for measurements.

My basic power arrangement is:

                 FE-5650A DB-9

15–18 V +  ------------------ Pin 1
15 V GND  ------------------- Pin 2
                                |
                                +---- Common ground
                                |
5 V GND   -------------------- Pin 5
+5.0 V    -------------------- Pin 4

Pin 3  ----------------------- LOCK output
Pin 6  ----------------------- Leave open
Pin 7  ----------------------- Signal ground
Pin 8  ----------------------- Leave open
Pin 9  ----------------------- Leave open

The seller specifies that the +15 V supply should be capable of handling a 2–3 A startup surge, with a steady-state current of approximately 0.5–0.7 A.

The +5 V supply requirement is much smaller, approximately 100–200 mA. I therefore use a regulated 5 V supply with adequate spare capacity.

Understanding the LOCK Output

This new information also explained some strange voltage measurements I had previously obtained from pin 3.

I had measured approximately 0.063 V on pin 3 during one test and later approximately 2.74 V. Initially, I had considered these voltages as possible indications of the lock state.

However, the seller confirmed that pin 3 is an open-collector LOCK/BITE output.

This means that pin 3 cannot produce a HIGH logic voltage by itself.

When the rubidium oscillator is locked, the internal circuit pulls pin 3 towards ground.

When the oscillator is unlocked, the internal transistor switches off and pin 3 becomes high-impedance, effectively leaving it floating.

Consequently, measuring pin 3 with a digital multimeter without a pull-up resistor can produce misleading voltages caused by leakage currents, the internal circuitry and the high input impedance of the meter.

My earlier readings of 0.063 V and 2.74 V therefore cannot reliably be interpreted as indications of lock status.

Adding the Pull-Up Resistor

To obtain a reliable LOCK indication, I need an external pull-up resistor between pin 4 (+5 V) and pin 3 (LOCK).

I chose a 4.7 kΩ resistor (4K7):

             Pin 4 (+5 V)
                   |
                 4K7
                   |
                   +---------- Pin 3 (LOCK)
                   |
              Multimeter +
                   |
              Multimeter -
                   |
             Pin 7 (GND)

I then measure the voltage between pin 3 and pin 7.

With the pull-up resistor fitted, the expected readings are approximately:

Unlocked: ~5 V

Locked: ~0 V

I must not connect pin 3 directly to +5 V. The resistor is important because when the FE-5650A asserts LOCK, it effectively pulls pin 3 to ground.

With a 4.7 kΩ resistor and a 5 V supply, the current is only about 1.1 mA.

Choosing the Correct Resistor

Resistor markings can easily cause confusion.

For example:

4R7 = 4.7 Ω

47R = 47 Ω

4K7 = 4,700 Ω = 4.7 kΩ

The letter replaces the decimal point and also indicates the multiplier.

For my FE-5650A LOCK pull-up, I therefore want 4K7, not 4R7.

The seller specifies a suitable pull-up resistance of 1 kΩ to 10 kΩ, making 4.7 kΩ a good choice.

Values such as 4.8 kΩ, 5.1 kΩ or 10 kΩ should also be suitable.

A 48 kΩ resistor is outside the seller’s recommended range, so I would not use one when a resistor between 1 kΩ and 10 kΩ is available.

It is also important to distinguish between powering pin 4 and providing the LOCK pull-up. Pin 4 receives +5 V directly and does not have a series resistor.

The 4.7 kΩ resistor goes between pin 4 and pin 3:

Pin 4 (+5 V) ---- 4.7 kΩ ---- Pin 3 (LOCK)

What Happens Without the Pull-Up Resistor?

If I omit the pull-up resistor between pin 4 and pin 3, I should not damage the FE-5650A simply by doing so. However, I cannot reliably determine the lock state by measuring pin 3.

Without the resistor:

Locked: the FE-5650A pulls pin 3 towards ground.

Unlocked: the FE-5650A releases pin 3, leaving it floating.

Therefore, an unlocked pin 3 does not necessarily measure 5 V unless I provide the external pull-up.

This explains why my previous measurements from pin 3 were difficult to interpret.

What Probably Happened During My Earlier Tests

During my earlier tests I supplied +15 V to pins 1 and 2.

The unit initially drew approximately 0.68 A, subsequently reached around 1.5 A, and became hot to the touch.

This suggested that at least some of the heater and rubidium physics-package circuitry was operating.

However, I had not supplied the required +5 V to pin 4.

The missing 5 V supply may therefore have prevented other parts of the electronics from functioning correctly. This could explain why I was unable to find the expected RF output when scanning from 1 MHz to 20 MHz and why the LOCK output behaved unexpectedly.

I therefore no longer regard my previous SDRangel frequency scan as good evidence that the FE-5650A was faulty.

Correct Testing Procedure

My revised testing procedure is:

1. With everything switched off, I connect +15 V to pin 1 and the 15 V supply ground to pin 2.
2. I connect regulated +5.0 V directly to pin 4 and the 5 V supply ground to pin 5.
3. I connect the grounds of the two power supplies together.
4. I connect a 4.7 kΩ (4K7) resistor between pin 4 (+5 V) and pin 3 (LOCK).
5. I leave pins 6, 8 and 9 disconnected.
6. I switch on both power supplies.
7. I measure the voltage between pin 3 and pin 7.
8. I allow the rubidium physics package time to warm up and watch the LOCK voltage.
9. Approximately 5 V on pin 3 indicates UNLOCKED.
10. A fall to approximately 0 V indicates LOCKED.

First Successful 10 MHz Output

After supplying the FE-5650A correctly with both +15 V and +5 V, I connected the RF output to my oscilloscope.

For the first time, I obtained a 10 MHz signal on the oscilloscope.

I also measured 5.1 V on pin 4, which is within the seller’s specified +5 V supply tolerance of 5.0 V ±0.25 V.

This is an important result because it demonstrates that the FE-5650A is now producing its expected 10 MHz frequency-standard output.

The remaining test is to establish whether the rubidium oscillator achieves lock.

With the 4.7 kΩ pull-up resistor installed between pins 4 and 3, I monitor pin 3 relative to pin 7:

Pin 3 ≈ 5 V   →   UNLOCKED

Pin 3 ≈ 0 V   →   LOCKED

The key event I am now looking for is for the LOCK voltage on pin 3 to change from approximately 5 V to close to 0 V as the rubidium system completes its warm-up and achieves lock.

At this stage, the fact that I have successfully obtained a 10 MHz output is a major improvement over my earlier tests and strongly indicates that supplying the previously missing +5 V rail was essential to operating this particular FE-5650A correctly.