An AD8307 Based RF Meter

The AD8307 IC is advertised as being a “Low Cost, DC to 500 MHz, 92 dB Logarithmic Amplifier”. This is a block diagram:
AD8307 Block Diagram

From the AD8307 datasheet: The essential purpose
of a log amp is not to amplify, though amplification is utilized to
achieve the function. Rather, it is to compress a signal of wide
dynamic range to its decibel equivalent. It is thus a measurement
device. A better term may be logarithmic converter, because its
basic function is the conversion of a signal from one domain of
representation to another via a precise nonlinear transformation.

And here’s a basic AD8307 circuit, mine is similar:
AD8307 Circuit

In my case, I have an LC filter on the incoming DC power, as well as the outgoing DC signal level, to reduce noise pickup. My meter is built into a small paint can, on the underside of the lid, which works as an excellent ground plane:
RF Meter

And here is the top of the lid, mounted on the can:
RF Meter
The toggle switch isn’t being used. I was going to power the meter off of a 9 volt battery to further reduce noise, but comparison tests between the battery and DC power supply showed no difference.

The output of the meter is a voltage proportional to the input power, measured in dBm. Zero volts is output for −84 dBm,
corresponding to a sine amplitude of 20 μV. There is a noise floor, and the specified range of the AD8307 is −74 dBm to +16 dBm. The output voltage increases by 25 mV for each dBm increase in RF input.


Previously, in Signal to Noise Ratios, I compared how the SNR affects the quality of the received signal, with some simulated recordings at various Signal to Noise Ratios.

I thought it would be interesting to also compare AM (Amplitude Modulation) vs SSB (Single Side Band) transmissions. While I’ve never been a huge fan of SSB (also referred to as Satan Side Band) for transmissions involving music, there’s no doubt that it does get out much better than AM.

Let’s take a look at the spectrum of an AM signal (click on it to enlarge it):

AM Spectrum

You can see the carrier on 9980 kHz, which consumes most of the transmitter power. Indeed, for a 100% modulated AM transmission, the carrier consumes half of the transmitted power. The carrier power is constant, so for less than 100% modulation (which is typical) the carrier is using more than half of the power. The carrier is necessary for demodulation of the sidebands at the receiver, but conveys no useful information.

To the left and right of the carrier are the lower and upper sidebands. They are symmetrical about the carrier, and convey identical information. Each has the same amount of transmitted power. For the case of 100% modulation, each has one quarter of the total transmitted power. For the typical case of less than 100% modulation, each has less than a quarter.

Next is the spectrum of an SSB signal, USB (Upper Side Band) in this case (click on it to enlarge it):

USB Spectrum

This station is transmitting on 13270 kHz. There is no carrier, and only one sideband is transmitted. Remember that the carrier consumes at least half of the transmitted power, and each sideband uses half of the remaining power, or one quarter for 100% modulation. So in the case of SSB, for 100% modulation, four times the power is available for the sideband as compared to AM, for a given total transmitter power. Four times is equivalent to 12 dB, or two S units.

As you can imagine, this is significant. I’ve created some simulated recordings of USB signals. For these simulations, I assumed that the typical modulation would be about 50%. A 100% modulated signal would sound louder (less noise, better SNR).

Listen to the simulated recordings below to see the effects of various Signal to Noise Ratios:
0 dB Signal to Noise Radio (SNR)
6 dB Signal to Noise Radio (SNR)
10 dB Signal to Noise Radio (SNR)
20 dB Signal to Noise Radio (SNR)
40 dB Signal to Noise Radio (SNR)

It might also be useful to compare them to the previously generated AM signals:
0 dB Signal to Noise Radio (SNR)
6 dB Signal to Noise Radio (SNR)
10 dB Signal to Noise Radio (SNR)
20 dB Signal to Noise Radio (SNR)
40 dB Signal to Noise Radio (SNR)

An AM signal with a SNR of 0 dB is almost impossible to listen to, while an SSB signal, while difficult, is intelligible.

These results suggest that homebrew 10 watt SSB transmitters would produce signals that could quite easily be received by listeners, in cases where an AM transmitter of the same power level would produce a weak signal with an SNR too low to be readily received. The problem, of course, is that SSB transmitters are much more difficult to construct. Ham transceivers are of course quite easy to obtain, and used ones are often relatively inexpensive (although not as cheap as the $30 or so it costs to build a grenade type transmitter).

Many operators run their SSB transmitters at full power, but it is possible that they would reach many of their listeners with lower power, possibly reducing the risk of FCC enforcement actions, if they are indeed related to power levels.

On a related note – why refer to SSB as “Satan Side Band”? While SSB is a far more efficient transmission method than AM, it does have one drawback. With an AM signal, being “on frequency” is not important. The transmitter and receiver frequencies can be off by hundreds of hertz, with virtually no impact on the received signal. The carrier is used in the demodulation (reconstruction of audio) of the signal by the receiver. As long as the carrier and sidebands fit within the receiver’s passband, the signal will be correctly demodulated.

This is not true with SSB. With SSB, there is no transmitted carrer. The receiver must produce it’s own carrier (often referred to as the BFO or Beat Frequency Oscillator in older radios). Ideally, the BFO frequency is exactly on the frequency of the missing carrier from the transmitted signal. In practice, there will always be an offset, due to neither radio being exactly on frequency. This offset is directly translated into an offset for all demodulated audio.

For example, if the radios are off frequency by 100 Hz, then all of the demodulated audio will be shifted by 100 Hz. For voice communications, this is not a serious problem. The speech can still be understood, and it is usually quite easy for the listener to adjust the received frequency until the audio “sounds right”.

The problem is with music. Here, even small tuning errors of ten Hz can cause the audio to “not sound right”. If you know the song in question well enough, you can adjust the received frequency until this error is reduced enough. There are two potential remaining problems, however:

First, many digitally tuned radios cannot tune with infinite resolution, or even in 1 Hz steps. Rather, they may be limited to a 10 Hz tuning step. 10 Hz is still too much of an error for listening to music. Some radios get around this by having a knob that can be turned to adjust the BFO in an analog fashion (often called fine tuning, etc).

The second problem is drift. If the transmitter is drifting around (or the receiver, or both), then the tuning knob will need to be continuously adjusted to bring the station back on frequency.

While I’ve always preferred AM over SSB due to the audio quality, there’s no doubt that watt for watt, SSB results in a much better SNR for the listener.

Signal to Noise Ratios

In a previous entry, How many watts do you need?, I discussed how transmitter power affects the received signal, and touched on the concept of the SNR, Signal to Noise Ratio. Seeing numbers expressed in dB is one thing, but actually hearing the difference between a station with an SNR of 10 dB and one of 20 dB is far more enlightening.

I created some simulated Signal to Noise Radio recordings. They were produced by mixing a relatively constant noise signal (actual static RF from a Software Defined Radio connected to an antenna) with a software generated AM modulated signal. One difference between these recordings and an actual station is that there is no fading, so real world conditions are likely to be somewhat worse, depending on the amount of fading the station is experiencing.

I’ve produced five recordings, with SNR’s of 0, 6, 10, 20 and 40 dB. A SNR of 0 dB means that the signal and noise levels are exactly the same. This is essentially the weakest signal that you could possibly receive. On the other hand, an SNR of 40 dB represents excellent reception conditions, say that of a local high powered MW station. The others obviously fall in between.

Remember that every 6 dB (voltage) of SNR is equivalent to 6 dB more signal (with the noise level held constant), in other words, doubling the transmitter power. Conversely, a drop of 6 dB is the same as cutting the transmitter power in half.

Let’s make up a crude example. A very strong pirate signal may have an SNR of 30 dB, somewhat weaker than a local station. Going from 30 dB to 10 dB, or 20 dB, is a change in transmitter power of a factor of 10 times. Going, for example, from 200 watt transmitter to a 20 watt transmitter. A 10 watt transmitter, half the power, would be 6 dB lower, or around 4 dB. It would be slightly weaker than the 6 dB simulated recording below.

Listen to the simulated recordings below to see the effects of various Signal to Noise Ratios:

0 dB Signal to Noise Radio (SNR)
6 dB Signal to Noise Radio (SNR)
10 dB Signal to Noise Radio (SNR)
20 dB Signal to Noise Radio (SNR)
40 dB Signal to Noise Radio (SNR)

DDS-60 Direct digital synthesizer

Recently I put together a DDS-60. DDS stands for Direct Digital Synthesizer. It is a way to generate arbitrary frequencies. Samples are fed to a D/A (Digital to Analog Converter) at a fixed clock rate (in this case 180 MHz derived from a 30 MHz oscillator). These samples are generated by a NCO (Numerically Controlled Oscillator). Think of it as a sine wave being generated point by point, at a fixed (depending on the ratio of the output frequency to the 180 MHz clock) number of degrees per sample. The output frequency can instantly be changed by just altering this degrees per sample value.

In the case of the DDS-60, any output frequency from 0 to 60 MHz can be generated. AD9851 DDS chip is used. This chip, along with a buffer/amplifier, low pass filter, and voltage regulator is all contained on a small (about one by two inch) board. The output amplitude is set by a small trimmer pot, with a maximum of about 4 volts peak-peak.

Three TTL level digital control lines are used to select the frequency. In my case, I have them connected to the parallel port of a PC.

I mounted the DDS-60 on the underside of the lid of a one quart paint can. The output goes to a BNC connector, there is also a 2.5mm barrel jack for 12V DC power, and a 9 pin D-SUB connector for the digital lines to the PC:

DDS-60 Board
There is a small LC filter (about 3 mH and 1000 uF) on the incoming DC power line.

DDS-60 board

Here is the resulting unit. Ugly, but it works!
DDS-60 in tin can

And here is the output on a scope:
DDS-60 output on an oscilloscope

So what can you do with a DDS?

First, it’s a very handy piece of gear for the RF test bench. You have a stable and precise source of RF that can cover the entire LF, MF, and HF bands. One of my next goals is to write some software to do automated testing and sweeps of RF, using an RF voltmeter as the input. I hope to blog about that shortly.

Second, you can use it as an exciter to drive an RF amplifier.

How many watts do you need?

Let’s say you’re a ham radio operator, or even a (gasp!) pirate radio broadcaster. How many watts of transmitter power do you need to reach your target(s)? Well, if you’re the typical ham, the answer is easy – just crank up the transmitter RF output knob to max. If you’re the typical pirate, you may do the same, although you’re a little more cognizant of the risks involved. Higher power is more likely to cause RFI issues with the neighbors’ TV, and possibly get you some unwanted attention from the FCC.

The alternative is to run low power. In ham lingo, this is called QRP. Most transmitters let you adjust your power level, so you can just dial it down. But to what level? How low can you go? What you’re trying to accomplish is to be heard by your listener(s). That is, the received signal is large enough to overcome noise levels, both from other signals and static, as well as receiver noise. The latter is a concern at VHF/UHF frequencies, but essentially a non-issue for HF, where atmospheric noise always dominates.

The signal to noise ratio (SNR) is defined as the ratio between the signal and noise levels, and is usually expressed in decibels (dB). 0 dB means the ratio is 1, the signal and noise power levels are the same. a 10 dB SNR means the signal power is 10 times the noise power, 20 dB means the signal is 100 times (it is a log based scale). These are for power values, for voltage ratios the SNR is twice the power value. A SNR of 0 dB would just be barely detectable, in practice you need a few dBs for even a weak signal, and a SNR of 30 or 40 dB is considered an excellent quality signal.

Noise levels vary tremendously, of course. Atmospheric noise varies with the frequency (higher at lower frequencies) and time of day (higher at night, when static from distant thunderstorms is more easily propagated). Then there are the potential man made sources of noise, such as other stations, as well as unintentional noise from the multitude of TVs, computers, switching power supplies, and so on, which have all contributed to a rise in the noise floor over the years.

There are many software tools to estimate received signal levels, based on transmitter power levels and propagation conditions, such as DX Toolbox. Plug in the numbers, and you can get an estimate of the received signal level. It might even be close – there are a lot of factors to consider, and many of them are unknowns, or at least estimates, such as solar effects on propagation.

Another way is to actually measure the received signal level. The good news is that most shortwave receivers have an s-meter, to tell you how strong a signal is. The bad news is that most of the time, the s-meter is wrong.

First, there is no concrete definition of how an s-meter should work. The ARRL suggestion is that an S9 signal is 50 microvolts at the antenna input, and that each S unit represents a 6 dB change in input voltage and power (that is, the voltage doubles, meaning the power level is 4 times higher). Tests on common receivers and transceivers show about a 1 to 5 dB per S unit change. That is, each increase in indicated signal level on the s-meter actually represents a smaller change in received power level, as compared to the theoretical 6 dB/S unit standard.

All other things equal, a change in transmitter power level causes a corresponding change in received power level. So if you double your transmitter power, the received signal will also double. According to the ARRL standard, increasing the transmitter power by a factor of 4 would add one S unit, in practice with most receivers it would add several S units, depending on what the original received signal level was. Again, the s-meter is just an indicator, the actual received signal level is what is important. Doubling the transmitter power will increase the signal, and SNR, by 3 dB. Likewise, cutting it in half will reduce the SNR by 3 dB.

So, let’s assume we have a transmitter running at 150 W (a pretty reasonable value for a good old fashioned tube rig like a Johnson Viking II). And let’s assume that the received s-meter reading is S9 dB, a very good signal, and it’s nighttime with a noise level, as indicated on the s-meter, is S4.

Here’s the Icom IC-730 S-meter sensitivity values from the previous link I gave:
S1 - 2 1.4 dB
S2 - 3 1.3 dB
S3 - 4 1.6 dB
S4 - 5 2.3 dB
S5 - 6 1.8 dB
S6 - 7 3.2 dB
S7 - 8 3.1 dB
S8 - 9 4.0 dB
S9 - S9+10dB 5.6 dB
S9+10dB - S9+20dB 7.3 dB
S9+20dB - S9+30dB 6.6 dB
S9+30dB - S9+40dB 10.5 dB
S9+40dB - S9+50dB 11.3 dB
S9+50dB - S9+60dB 13.5 dB

Ok, so let’s see what happens as we reduce the transmitter power. Each time we cut it in half, we reduce the received signal by 3 dB. Reducing it to a quarter would be 6 dB, an eight would be 9 dB, and a tenth would be 10 dB. Got it?

Looking at the chart, going from S8 to S9 is a 4 dB change. That would correspond with reducing the transmitter power by a factor of 0.40, or down to 60 watts. Going from S8 to S7 is 3.1 dB, a power reduction to 0.49, or 29.4 watts. S7 to S6 is 3.2 dB, a factor of 0.48, or down to 14.1 watts. S6 to S5 is a factor of 1.8 dB, 0.66, or 9.3 watts. And finally S5 to S4 is 2.3 dB, a ratio of 0.59, or down to 5.5 watts.

So what does all this mean? Well, if we dropped our transmitter power from 150 watts to 5.5 watts, the received signal would drop from S9 to S4. We stopped there because the noise levels were S4. At this point, the signal is barely audible. At 9.3 watts, pretty close to the magic 10 watts that most grenade type transmitters put out, the received signal is S5, one S unit above the S4 noise, 2.3 dB above, so an SNR of 2.3 dB. Something you could listen to, but it would really be down in the noise.

What about the original 150 watts that produced an S9 signal? Well, let’s just add up our dBs. 2.3 + 1.8 + 3.2 + 3.1 + 4.0 = 14.4 dB. So in this case, the SNR is 14.4 dB. Not the 20 or 30 dB you’d expect from say the BBC, but certainly pleasant enough to listen to.

Obviously this is just one example. With different assumptions, especially noise levels, the results will be different. Much lower noise levels would allow weaker transmissions to be heard. If the noise was S1 instead of S4, that’s 4.3 dB of SNR right there. Likewise, higher noise levels intuitively imply more transmitter power is necessary. But I think these are reasonable assumptions for nighttime noise levels on 43 meters, and typical pirate transmitter power levels.

The numbers speak for themselves. The difference between the received SNR for a 150 watt and 10 watt transmitter is huge. Of course, as the difference between getting the knock and not is also huge. Assuming transmitter power levels have an influence on FCC enforcement activity…

A 30 mW unlicensed CW beacon was busted last year:
Last summer the F.C.C. DFed the Echo beacon that was on 11002 Khz. It had been running 30mW.
The FCC agent was kind and considerate and dropped further investigation. In fact he was respectful of the fact that it was
completely Homebrew and will under 100mW. There was no complaint
and the only one who cared about the beacon was the DF site in Maryland. The little beacon was disconnected and dismantled.
The operators pirate beacon days are over.

20 to 30 mW is extremely lower power, even for CW. This tells us two things: first, the FCC has very good ears, and can pick up weak signals. If they can pick up a 20 mW beacon, they can easily pick up your 10 watt grenade. Second, and more importantly, they are probably more concerned with the frequency the unlicensed station is using, than the number of watts. If you look through recent busts of pirates, you will see that they are mostly due to choice of frequency, as well as the unusual bust of Weather Radio, which appears to have been motivated by their use of the National Weather Service’s DECtalk speech synthesizer voice.

This is not to say that transmitter power plays no role. But it may not be the FCC’s primary enforcement trigger. The FCC is Complaint Driven. A scan through their Enforcement Bureau confirms this. Busts are mostly for FM pirates, likely based on complaints from licensed stations, as well as for other offending transmissions, such as those that interfere with cellular phone service.

Busts of HF pirates would also likely be due to complaints from licensed services, especially the military, which does use parts of 43 meters. Those BLEEP BLEEP BLAPPP sounds you hear are the TADIL-A/Link 11 system. I can imagine that 70s pop music or cut and paste audio loops interfering with them don’t go over well with the men in uniform. One call is all it takes for the offending pirate to suddenly be #1 on the FCC’s enforcement list.

Back in the 1990s, The infamous pirate Voice of the Night, operated by Lad, was QRMing a Havana/Moscow CW net on 7415 kHz. The operators could often be heard sending strings if FUFUFU in CW in response. Apparently it also annoyed certain radio monitors employed by No Such Agency, as Lad was quickly busted.

So worry about how many watts you’re sending into the ether, but also worry about your choice of frequency.