There are three things to consider when configuring an external coupler and/or external attenuators to obtain a proper feedback level at the rear panel connector of the radio:
1. When using Puresignal, the Auto-Attenuate function drives the step attenuator to the setting necessary to obtain approx. a feedback signal level of +3dBm at the ADC input. This is -17dBm at the output of the step attenuator for all radios except the 7000 and 8000 due to the 20dB preamp used prior to the ADC. The 7000 and 8000 both use a 14dB preamp, so this is -11dBm at the output of the step attenuator in those radios, both of which use the newest Orion MkII SDR board.
2. On older radios, step attenuator levels of below 7dB or so generally result in poor performance because internal crosstalk dominates the external feedback signal. The newer the radio, the less of an issue this is. However, better to have it (a higher feedback signal level) and not need it then need it and not have it.
3. It generally pays to set levels such that the step attenuator obtains a rather high setting, say in the 25dB range when RF output power is at maximum. There are two reasons for this. First, this provides a good dynamic range situation such that, when operating at reduced power levels, Auto-Attenuate has room to reduce the step attenuator value when not QRO. Second, there is a better feedback signal to crosstalk ratio on the PA/RF board as discussed in (2) above.
With these three guidelines, one can easily calculate the coupling factor and/or any additional attenuation in the feedback path necessary to obtain the desired step attenuator value and proper feedback levels. For example, let's consider a typical legal limit scenario of an external amplifier at 1500W peak RF power and an Xtronic coupler with 44dB coupling factor, and a desired step attenuator setpoint of 25dB.
Step 1--calculate peak RF power in dBm. Power in dBm = 10log(power in Watts)+30. In this case 1500W = 61.76dBm.
Step 2--calculate output power level of the coupler at peak RF power input. 61.76dBm - 44dB = 17.76dBm.
Step 3--calculate required input power to the step attenuator to obtain -17dBm at the output of the step attenuator at your desired step attenuator setpoint. -17dBm + 25dB = 8dBm (assumes any radio except the 7000 or 8000, just change -17 to -11 for those radios).
Step 4--calculate the additional external attenuation required between the coupler output level determined in (2) above and the rear panel input to obtain the feedback level determined in (3) above. 17.76dBm - 8dBm = 9.76dB.
Step 5--beg, borrow, buy, make an attenuator of the value identified in (4) above and install it between the coupler output and the rear panel input (Bypass on newer radios using the Rev. 24 PA/RF board or the 7000, Ext1 on older radios using the Rev. 15 or 16 PA/RF board, PS Input on the 8000).
Of course you can simply use an attenuator that is close in value, it need not be exact. The step attenuator will make up the difference. However, do not use too low of a value, as a) you do not want to exceed +13dBm at any rear panel input otherwise you risk damaging the radio, and b) the attenuator will run out of dynamic range, 31dB is the maximum it can be set to.
One thing that you might consider is obtaining a rotary step attenuator with 1dB steps. They are usually available on eBay for not very much money compared to fixed attenuators. This way you can dial in just the right amount of additional attenuation you want or need.
