HP 415E SWR Meter

© Brooke Clarke, N6GCE

Front Panel	Inside & rear	Inside & Front of 2709A10766	A3 PCB 00415-60023

Background

 Back in the days of tube type equipment there was an HP 415 SWR (Standing Wave Ratio) meter and there have been a number of versions ending with the latest "E" model.  This instrument is a very narrow band 1 kHz attenuator and amplifier and a precision rectifier.  The relative amplitude of the 1 kHz signal can be read to scale ticks of 0.05 dB and interpolated to 0.01 dB.  There is also a scale for the original purpose of reading SWR.

Note that if Full Scale is set on the "zero" range then when reading down 10 dB on the 60 range you are reading a signal that's down 70 dB.

The 415E is 6" high and 7 3/4" wide.  This does not appear to be a standard Rack Mount instrument size.

Model History

Controls & Connectors

Front Panel

The analog meter movement is a mirrored type where you look for the reflection of your eye's pupil bisected by the needle and thus eliminate parallax in the measurement.  There is also a mechanical meter zero pot.  The correct way to use this is to turn the pot in one direction so that the needle moves up scale and and then comes back down scale and you stop turning when the needle is over "2" on the expanded scale.  If you over shoot, do not turn the screw in the opposite direction, but rather, continue turning and approach the zero position from up scale.

The POWER switch has positions for OFF, ON, (and for instruments with the optional internal battery supply: Battery: ON, TEST and CHARGE).  

The INPUT switch has positions for XTAL IMPED: HIGH, LOW or BIASED (+1.3 VDC on the center contact with 12.6 mA of current capability) and BOLOMETER: 4.5 mA or 8.7 mA.  To match a wide variety of input sources.  The HIGH position is also used for signal sources that are not from a detector or power sensor.

The INPUT is a BNC(f) connector with the shell 50 Ohms above ground.

The FREQ adjust pot allows the center frequency of the 415 to be set to match the frequency of the source generator.

The BANDWIDTH control adjusts the width of the passband and therefore also controls the signal/noise ratio for faint input signals.  If the 415 is used with a sweeper this control should be set fully CCW to increase the BW to allow for signal variations as the sweep progresses.  Also the amplifier output should be used to drive a scope or chart recorder instead of the recorder output since the latter has a low pass filter that limits it's bandwidth.

The RANGE-DB control has an outer knob with positions for 0, 10, 20, 30, 40, 50, and 60 dB a very large dynamic range, and an inner EXPAND knob with positions of: NORM, 0, 2, 4, 6 and 8 dB.  By using the expanded (red) scale positions you can read amplitude to 0.01 dB over the full 60 dB range.  On the 0-NORM range you can bring the needle to full scale with an input signal between 0.120 VAC and ____.

Comparing the 34401 DVM with the 415E shows that the noise bandwidth of  the 34401 limits its use:

415E	34401A
FS 10 range	1.4 V
FS 20 range	140 mv
FS 30 range	14 mv
FS 40 range	1.4 mv
FS 50 range	0.01 mv*
FS 60 range	can not get signal this low

* 34401 is readng noise and 415E has a Full Scale reading.

The Gain & Vernier controls will bring the needle to Full Scale with an input of more then 0.12 Volts up to more than 5 Volts.

Square Law

Note that the 415 reads 10*LOG(voltage ratio), this is called "Square Law" and when the 415 is fed from a microwave detector that has a low level signal at it's input the 415 will read the same as a power meter becasue the detector has a "square law" response.  If you were trying to measure the dB difference in two voltages you would normally use dB = 20 * LOG(voltage ratio), but that's not what the 415 was designed to do.

Rear Panel

On the rear panel there is a BNC(f) RECORDER output that has a 0 to 1 VDC signal to drive a chart recorder and a pair of banana jacks on the standard 3/4" centers that has the amplified 1 kHz signal.  There is a covered hole that can be used to change the input connector from the front panel tothe rear panel.

Operation

I used the 415E for making microwave Smith Chart plots in conjunction with a slotted line and for making attenuation and/or gain measurements using a crystal detector.  The 415E is very good for both of these measurements and probably is the highest dynamic range scalar measuring instrument around.  It would also work fine for making optical measurements with an optical to DC converter.

I'm considering using it to measure the output of magnetic field sensors, where it's narrow noise bandwidth may greatly extend the sensitivity of the sensor.

When a 5.04 k Ohm resistor is connected to the input and the range set for Xtal Imped High and both the gain and vernier knobs are turned fully clockwise the meter reads 6.5 dB below full scale.  The manual addresses the theoretical KTBR noise of a 5 k Ohm resistor and how the 4 dB noise figure of the 415E will show up in the measurement.  This shows how sensitive the meter is.

Typical uses for the 15E are: SWR measurements using a slotted line, Power measurements using a bolometer, Relative power measurements using a crystal detector.  In all cases the source must have a 1 kHz square wave modulation. 

Manuals

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