A quick look at the QB-300 amplifier

Available from many surplus sellers (e.g. EvilBay) - and (usually) for a reasonable price - is the QB-300 RF amplifier.  Originally made by Q-Bit corporation, this same device has borne several different manufacturers over the 30+ years since it was introduced - but it is (pretty) much the same device.

Figure 1: The BNC-connectorized version of the QB-300. This appears to be the "original" version, actually made by Q-bit Corporation.  The voltage specification is slightly ambiguous, being shown as "+15/24 Vdc". Click on the image for a larger version.

Having several of these on-hand I decided to take a quick look at its apparent performance - with the general specifications for this device being conveyed below for your convenience:

• Frequency range:  1 MHz-300 MHz
• Gain:  23dB (24.5 +/- 1 dB)
• Gain flatness:  1 dB
• Noise figure:  3.8dB
• Input/Output VSWR:  <=1.5:1
• Power output (1dB compression):  +22dBm
• 3rd order Intercept:  +37dBm
• Current consumption:  155mA

Depending on which data sheet you consult, there are a few discrepancies - for example:

• The data sheet from "API Technologies" shows the input/output return loss as 1 dB - clearly a typo.
• The maximum voltage rating is all over the map:  Some versions of the data sheet show a maximum of 20 volts, others show 24 volts while the units that I have clearly show the voltage rating as being "+15/24Vdc" and the equipment from which it was pulled provided 24.0 volts.

Knowing the provenance of this equipment, I would have no problem running this amplifier from 24 volts, but based on the ambiguity of the data sheets, I might operate a version that did not specify 24 volts at between 15 and 18 volts.

A quick test:

Curious about a few aspects of these amplifiers, I decided to test it with my DG8SAQ Vector network Analyzer, checking its gain versus frequency - the results being displayed in Figure 2, below:

Figure 2: A sweep of the amplifier from 100 kHz to 500 MHz showing the apparent gain and input matching over the frequency range.  Because the DG8SAQ and the interconnecting cables are increasingly imperfect with increasing frequency, expect increasing uncertainty in the S11 readings above 100 MHz or so. The gain, S11 and VSWR values at specific frequencies can be seen in the upper-left corner. Click on the image for a larger version.

Of particular interest was the usability of this amplifier above and below its "official" frequency range - and we can see that it's probably useful down to at least 250 kHz and above 450 MHz, albeit at reduced performance (e.g. lower gain, maximum output power, increased noise figure, increased input VSWR.)

Gain versus operating voltage:

Figure 2 was captured with the unit operating at 18 volts and readings were taken at lower voltages, comparing the gain - but your mileage may vary:

• Gain dropped by approximately 0.1 dB at 15.0 volts.
• The gain was about 0.2 dB lower at 12.0 volts than at 18 volts.
• The gain was about 1 dB lower at 8.0 volts than at 18 volts.
• The gain was about 5 dB lower at 5.0 volts than at 18 volts.
• The amplifier began to exhibit signs of low-frequency instability below 5 volts.

Although not directly measured, one should expect the maximum output power (P1dB) and the intercept point to drop below the specifications below 15 volts:  The amplifier is likely to be perfectly usable in the 12-14 volt range, but it's likely marginal at 8 volts.

A peek under the hood:

Popping the top cover, we see this:

Figure 3: A look inside the QB-300 amplifier:  The input and output is on the left and right sides, respectively. Click on the image for a larger version.

It is immediately apparent that this is not a run-of-the-mill consumer device:  Rather than a circuit board, the unit is built onto an alumina substrate with both soldering and welding being used.  Two RF components are obvious:  The black, 3-lead device near the upper-left corner and the white ceramic device marked with "Q-21" just to the right.  The rest of the components are likely related to feedback/equalization as well as regulation of the operating and bias voltages for the RF devices.

Clearly, it's not hermetically sealed or conformally coated, so  weather protection is certainly warranted if this were to be operated outside.

Uses for this amplifier:

This amplifier was designed as a general-purpose gain block in the HF-VHF range, but it is likely useful into the low UHF range meaning that it should work from the 630 meter amateur band (on the low end) into the 222 MHz - and possibly the 70cm - amateur bands on the high end.

For general HF (amateur radio) amplification purposes, it should be an excellent performer - provided that one keeps in mind that a signal input level of a bit less than 0 dBm will push it into overload - and such off-air signals might appear from:

• Local AM broadcast stations.  Especially on a long wire antenna (longwire, rhombic, end-fed half-wave) these signals can, by themselves, overload the amplifier if you live anywhere near  a transmitter.  A simple high-pass filter can effectively reduce such signals and prevent overload.

• High-power shortwave stations.  On a good antenna, signals on the 49, 41 and 31 meter band can be extremely strong in Europe and some parts of the U.S.

For receive-only purposes it is often the case that a good receive antenna will not have much gain - and here are just two of the many possible examples:

• Small receive loop.  This type of antenna - usually around 3 feet (1 meter) diameter can offer local noise rejection as well as the ability to null signals.  This type of antenna will have negative gain but its performance can be quite good with a decent, low-noise amplifier like the QB-300.  For an antenna like this, one would place the amplifier at the antenna to minimize cable losses.

• Beverage on the ground.  Also known as the "BOG" antenna, this is simply a wire - as long as possible - laying on the ground against a good (and electrically quiet) ground.  This antenna - mostly useful in rural areas - is reported to work well overall.  Like the receive loop, it's best to place the amplifier at the antenna feedpoint.

It should go without saying that any amplifier connected to a large antenna should be preceded by adequate lightning protection to prevent damage to the amplifier from wind static/discharge and nearby lightning strikes as depicted in Figure 4, below.  Decent protection can be had with four ordinary silicon diodes - two series pair connected anti-parallel (back-to-back) with a bleed resistor (4.7-100k) to shunt voltages above about 1.2 volts.  (Note:  The amplifier itself would overload before the conduction voltage of two series silicon diodes.)

Figure 4: Depiction of simple input protection circuitry. On the left, the diodes ("D") are ordinary silicon diodes connected in series for approximately 1.2 volts of conduction. On the right, a common full-wave rectifier module is used with its DC "output" shorted, providing an equivalent to the circuit on the left.  It is suggested that a low current (2-5 amp) rectifier be used. The voltage rating of the diodes is not particularly important - a 50-100 volt rating being just fine. Resistor "R" is not critical and can be anything from 4.7k to 100k and it is used to dissipate any accumulated DC.  An inductor can be used in addition to/instead of "R" - a value of 22-100uH being suitable for 160-10 meters. Click on the image for a larger version.

Provided that one avoid excessive signal input level, it can also be used as the basis for a receive multi-coupler.  For example, following the amplifier with an 8-way RF splitter - which, itself, will have a loss of around 10dB - the overall gain will be in the range of 14dB while preserving the system's overall noise figure to allow reception of weak signals on the higher HF bands.

This page stolen from ka7oei.blogspot.com

[End]